Personality & Character
Reiji is a mysterious boy who Sei Iori encounters in town. He's overbearing, selfish, quick to anger and lacking in manners. Because he is from a different planet, Reiji has shown a naivety with Earth customs, such as comparing Rinko Iori and China Kousaka's breast sizes out loud, ending with him getting slapped, and taking food he thought was discarded from a shop, solely because it's placed outside. However, he has shown a noble side, helping others in their times of need. Despite him being inconsiderate, he is not totally devoid of social sense, shown when he chastises Mao for the failed date due to Mao being too straightforward, and that Mao should not have attempted to kiss Misaki right from the start.
Reiji initially has no idea about anything regarding Gunpla or Gunpla Battle, and was only into it for returning Sei's favor and that it seemed fun to defeat other Gunpla. Over the course of the time he spends on Earth, he gets more and more interested in Gunpla due to Sei's influence and other players around, eventually culminating in him building his own Gunpla - a GPB-X80 Beginning Gundam.
Arriving on Earth and meeting Sei
On a high construction platform, Reiji is seen commenting on how amazed he was by the number of buildings that people stay in on Earth, and mentions that it is several times larger than Arian. Soon after, Reiji encounters Sei Iori for the first time, asking him about the commercial of the "7th Gunpla Battle World Championship". This shocks Sei because even if Reiji was a foreigner, it was uncommon to not know of "Gunpla Battle". During the confusion Sei's stomach growled, so Reiji attempted to offered him some bread, but the store owner appeared making it clear Reiji stole it from his shop, though it was made clear that he had unintentionally stolen from the bread shop. Sei covers for the slip up since Reiji does not have any money, and in gratitude Reiji gives Sei his second Arista Gem and promises that he would repay him and that he would come if he were to wish anything on that stone.
Later, when Sei struggles to beat Sazaki's Gyan, his wish to avoid losing transmits over the Arista link, which led Reiji to appear out of nowhere and take control of the Build Strike's battle cockpit. He shows amazing piloting and battle skills despite never having interacted with a Gunpla Model or participating in a Gunpla battle and is able to pull off highly advanced maneuvers after only fiddling with the controls for seconds, leading Mr. Ral to compare Reiji to a Newtype. After the battle he disappears from the room without a trace, prompting Sei to eagerly run in search of Reiji, the person worthy of piloting his Gunpla. But Sei returns to his house only to find Reiji eating Rinko's vegetable stir-fry, much to Sei's shock. 
Sometime afterwards, Sei offers Reiji the chance to control his Gunpla in the World Championships due to his amazing skills, but Reiji refuses as Gunpla battle didn't interest him, fighting Sazaki's Gyan was merely repaying a debt, Reiji goes back to sleep. After waking up, In the morning, Reiji asks Sei's mom where Sei was, so she told him that Sei was at school and even went as far as to explain what is a school. This factor confused Reiji making it obvious that either in Arian there were no schools or it was because of his noble lifestyle.
Out of boredom and curiosity Reiji decides to go to Sei's school, and ended up engaging in battle with Monta Gonda. After a swift victory, Tatsuya Yuuki barged into the arena and easily defeats a weakened and incomplete Build Strike with his Zaku Amazing. After which he taunts Reiji and Sei into completing the Gundam and avenging their defeat in the Qualifications. Reiji agrees to work with Sei because of how embarrassing the one-sided defeat was, but suddenly Reiji begins to glow and then mysteriously disappears in a flash of red light. 
Reiji beats Mr. Ral's MS-07B Gouf at the store, Mr. Ral is impressed but reminds him that his own strength didn't win the battle but was due to the performance of Sei's Gunpla too. Reiji responds by calling Mr. Ral's Gunpla obsolete. Sei then enters the store and demands to know who Reiji is. At the time, Reiji claimed the existence of a kingdom know as Arian, and that he was the prince of said kingdom, having found his way to Earth among the castle treasure and decided to stick around after enjoying Gunpla. Sei does not believe him, mainly because Reiji portrays himself as a kind and loved prince, but also because there are no known planets, other than Earth, with life. He gives up on finding more about Reiji's past and decides to focus on Gunpla.
Tatsuya Yuuki never actually battles them in the tournament, having withdrawn before their battle due to unexplained reasons. This leads Reiji to be disappointed and he almost gives up the game. He later met with Mr Ral, who offers him some encouragement. He then met Tatsuya who invited him to a battle at the school battle arena. Thanks to the battle, both of their Gunplas took massive damage, utterly being destroyed and forcing both parties to continue their battle with their respective booster units, which too were later destroyed. Reiji and Sei lost the match again, and since time was too short to have Sei rebuild the Build Strike, they resorted to finishing the Tournament with their spare Gunpla, the Build Gundam Mk-II.
Having been victors at the qualification rounds, Reiji went with Sei's family, China and Mr. Ral to a hot spring inn near a beach as part of their winning prize. There, he discovers the sea water is actually salty, something that is true in possibly every beach in Earth, tough Reiji points out he has been to a beach, but without salty waters. This somehow supports an ongoing theory of Mr. Ral that Reiji might live in a colony (Again, there is no way of telling if Ral was joking or was being serious, though it is possibly both). Later, he and Mao Yasaka tag teamed to protect the inn from being bought out by defeating an Apsalus III built by Tatsuzo, a Land Shark.
7th World Championship
Reiji enters the World Tournament with Sei, where he develops a friendly rival relationship with Aila Jyrkiäinen of team Nemesis due to their shared love over food. Neither were aware of the other status as competitor in the tournament although both acknowledge their Gunplas as worthy opponents. Sei and Reiji's improved Star Build Strike draws the attention of Nils Nielsen who had formerly met them in a girls tournament and Luang Dallara, the Thai competitor who easily figures out how to counter it. The duo also met Yuuki under the new Alias Meijin Kawaguchi.
In the second match (Survivor Battle Royal), Reiji and Sei formed an alliance with Mao Yasaka and Ricardo Fellini while attempting to protect Fellini's Wing Gundam from a bunch of competitors whose girlfriends the Italian Competitor had stolen. Ricardo previously helped Reiji to get familiar with the Gundam Battle controls through a series of battles in Ral's secret fighting society club.
Their success at destroying a party of superior number exposes Reiji's face to the CEO of PPSE, Mashita, who correctly assumes's Reiji to be the crown prince of Arian. This causes the CEO to panic, which in turn made Mashita target Sei and Reiji's Gunpla in order to get them out of the competition.
Unaware of being targeted by the tournament sponsor, Reiji and Sei had to deal with a programmed Mega Size Zaku, which took the combined efforts of Reiji, Mao, Fellini, Nils and Kawaguchi to defeat. Following that, Baker, PPSE CEO's PA, rigged the matches so that the two had to fight Luang Dallara in a baseball field with only a glove as weapon but this plan was foiled by Dallara's integrity and Sei's RG System.
Reiji competed in fourth round target match (rifle shooting) using his backup Gunpla (the Build Gundam Mk-II) with assistance from Sei. They were also able to win the fifth round Tamarie contest. On the sixth round (3-on-3 battle), Mao Yasaka ensured their victory by disabling the opposing Gunplas with his GX-9999 Gundam X Maoh's Satellite Cannon.
Mashita almost succeeded in eliminating the threat by assigning a Gunpla Mafia Assassin, C, to capture and destroy the Strike during the seventh round race. The destruction of the Gunpla was averted by the interference of Mr. Ral, who confronted C and Sei's use of the RG system to escape, however they were unable to finish the race (having lost their boost while the Renato Brothers kept theirs and ended the race) and were thus bumped to 17th place, one less than the number of finalists.
Having recognized C, Mr. Ral warns the boys about the hidden danger. Sei then remembers the Mega Zaku (whom PPSE had implanted as a Bonus Trap in the Battle Royal) had also focused its attacks on them. This motivates Reiji to barge into the Chairman's mansion and confront Mashita, to the horror of the latter. During their talk, Mashita recognizes Reiji's Arista Bracelet, eliminating any doubts he had about Reiji not being the Crown Prince of Arian. However, since Reiji did not recognize Mashita, he was able to excuse himself from the tampering suspicion.
Following that incident, Mashita admits to Baker (and to Nils Nielsen, who was spying on them) that should Reiji remember who he is, the secret of his past and of the Plavsky Particles will be endangered. Apparently, the CEO opts out of interfering with the eight qualification match, possibly to prevent further provocation on Reiji since the latter seems more concerned with ensuring the integrity of their fights.
In the last qualification round (single battle), Reiji and Sei's Star Build Strike duels against Italian Champion's Ricardo Fellini XXXG-01Wf Wing Gundam Fenice. A long time ally of the pair. Even though Fellini was already qualified, and thus had the option to surrender the match in order to promote Reiji (whom Ricardo treated as a pupil) and spare his Gunpla from damage, he instead chooses to go all out in the match at the cost of extensive damage to both Gunplas (much like the battle with Tatsuya which disabled the Full Package). The match ended in a draw with both Gunplas disabled at the same time, however Reiji and Sei got two points (out of the assumed four points given to the winner of the match) which enabled them to reach the sixteenth position for the finals.
Break before the Finals
Reiji and Sei argued after Sei refused to let Reiji assist him in rebuilding the GAT-X105B/ST Star Build Strike as Reiji had no experience building Gunplas. He decided to prove Sei wrong by crafting one on his own. At the Shop he met Takeshi Iori, Sei's father, under disguise. Takeshi selects a GPB-X80 Beginning Gundam for him to build and picks a Command Gundam for Aila, who had been spying on them.
Reiji and Aila, under strict coaching from Takeshi, were able to build a fairly high quality models despite those being their first attempts. Reiji decided to test it in battle against the Store AI since Aila refused to battle (claiming not to know how to). Unknown to him, Mafia Member C had hijacked the system and input his Hell Zeong Galaxy Gunpla to fight and destroy Reiji's Gunpla (not caring that this would have amounted to nothing as far as his contract with PPSE went).
The damage done to Reiji's Beginning Gundam led him to understand Sei's own feelings towards protecting the integrity of the Build Strike, and the amount of time and effort needed to build and maintain a unit as high as that of Sei's. Reiji's Gunpla would have been destroyed had it not been for Aila's interference in the battle. They fought exceptionally well together and totalled C's machine, Allowing Takeshi, also a International Gunpla Referee to arrest C. After the battle Reiji and Aila held hands and he questioned her about her ability, to which she denied having previous experience in fighting.
Reiji later presented his Gunpla to Sei, Mr. Ral and China. He admitted to having help, showing the clippers he had been given by Takeshi, which Sei recognized as being his father's.
7th World Championship Finals
Reiji and Sei arrive at the arena (late) to see the semi-final matchups. After arriving, they notice that their next opponent will be Yasaka Mao. Mao immediately greets them, warning them of their imminent defeat at his hands. Reiji and Sei spend the day preparing the Star Build Strike for the battle, and Reiji explains that Mao is simply afraid of losing.
After Mao returns to the arena ready to fight and the battle commences. Gundam X Maoh immediately opens fire upon the Star Build Strike with its hyper satellite cannon. Sei is then shocked at how quickly the satellite cannon can fire a second shot. The second shot manages to destroy the absorb shield, therefore limiting the fighting strength of the Star Build Strike. Reiji immediately notices that Mao is no longer distraught by the fear of losing, and is now completely immersed in the battle itself. After a quick exchange which results in the destruction of the hyper satellite cannon, as well as minor damage to the star build strike, the final stand-off commences. Mao combines his solar system, with power from the satellite system in order to create a large beam saber (Maoh Sword). Reiji activates the RG system, and uses the Build Knuckle to effectively counter the Maoh Sword. The build knuckle proves victorious and pushes through Maoh's attack, destroying the Gundam X Moah in a large flash.
After the battle, Sei and Reiji rest by the side of the arena after the match. While talking, they notice the next battle has already ended. After rushing over to see the result, they witness Luang Dallara in a shocked state. It appeared that the Renato brothers had beaten him easily (thanks to the use of diversion and sabotage tactics), and Reiji stood there in awe of how Luang was defeated.
Reiji is also present to watch the match between Meijin Kawaguchi and the Renato brothers. Throughout the match, Reiji is surprised at how well the Renato brothers were fighting against Meijin's Kampfer Amazing. He is even shocked when the Kamper Amazing is damaged and is close to losing. Reiji is however relieved to see Meijin pull a comeback by using amazing strategy and timing.
When Nils Nielsen uses his Sengoku Astray to battle Georgios Laertius's custom Gundam Virtue. Sei and Reiji watch in amazement as the Sengoku Astray easily cuts through the Virtue's beam barrage and wins the battle. Later during the day, Reiji and Sei went to meet Nils upon his request.
Nils was looking to find out the secret of the plavsky particles and assumed, after overhearing the Chaiman and Baker, that either Sei or Reiji were aware of the secret. He offered to withdraw from the match if Sei or Reiji gave him the answer. Sei got angered when Nils called Gunpla Battle "kids playing with toys", and Reiji backed him up giving Nils no option but to fight his way to the secret.
The next day, Reiji prepares to face off with Nils. The match is set on a castle field, with Reiji rushing into battle. Nils easily cuts through his beam shot, but this was anticipated by Sei. The Star Build Strike immediately sheds its booster, beam rifle, beam sabers and absorb shield to decrease its weight and as they are all useless against the Sengoku Astray. Reiji then activates the RG system and the Build Knuckle, engaging in close quarters combat with the Sengoku Astray. Although Nils seems overly confident as usual, he is shocked to see his Samurai Swords destroyed by the build knuckle. As Reiji attacks once again, Nils activates his famed Particle Fa Jin and destroys the Build Strike's right arm instantly. However, this was not without a cost, as the Sengoku Astray also lost its right arm. The two machines attack each other once again, but a stalemate is once again the result.
Before Reiji can proceed to attack again, the match is halted upon Baker's order and this give both fighters a chance to repair their Gunpla. Before the time runs out, Sei, with some assistance from Reiji's newfound ability in building Gunpla, manages to fix the Star Build Strike and it launches into battle once again against the Sengoku Astray. By putting instant glue on the Build Strike's right arm Sei managed to make the Particle Fa Jin innefective. In an intense exchange, both machines attack for the last time, this time with the Star Build Strike using perfect timing to avoid the Sengoku Astray's attack and land the final blow. The episode ends with Reiji and Nils shaking hands after Nils admits he enjoys Gunpla Battles.
While Ricardo Fellini is preparing for his match against Team Nemesis, Reiji is shown relaxing in his room. Sei, Nils and Mao are also there, with Nils attempting to hypnotize Sei into remembering the secret (unaware that it is Reiji who knows it). This attempt at hipnosis leads Sei to recite all episodes of the original Gundam Series, which makes them arrive late to the match between Fellini and Aila.
They arrive just in time to witness Fellini fall against the Qubeley Papillon. Reiji is surprised to see how badly damaged the Wing Gundam Fenice is, and how the Qubeley is only slightly damaged. They watch in awe as Fellini tries to self-destruct as a last resort. After he abandons this plan on advice from Kirara, the Qubeley is seen repeatedly stabbing the Fenice with its lance, as Aila has no control over her actions due to overuse of the Embody system.
Reiji quickly rushes into the battle with his Beginning Gundam. Enraged by Aila's actions, Reiji attacks the Qubeley, but the Beginning Gundam is quickly crippled and defeated. Reiji stands shocked for a moment as he faces his defeat. He then rushes to confront Team Nemesis for their actions, but is immediately halted when he sees a part of Aila's face through her broken helmet, revealing her true identity. The fact Aila had lied to him recuringly angers him, to which he swears, on the pride of the Arian Royal Family, that he will defeat her in the Semi Finals.
Mashita, who was already well aware of Reiji's identity, is further troubled by Reiji's statement as up to this point, despite all evidence, Reiji had yet to claim to be the Prince whose existence the chairman so much fears. Baker and him do not tamper with the fight believing Aila will be enough to defeat the boy.
Concerned over Reiji's fury with Aila, and Aila's own reasons to fight in the tournament so viciously, Sei develops a plot in hopes that the two Ace Fighters can clear the air over them. He asks China to gift Aila with the Arista stone he had kept on his GP Base, now crafted into a necklace, in hopes that Aila's own wishes will reach Reiji.
Aila is hesitant to fight Reiji, to the point she refrains from dealing critical damage to the Star Build Strike. This causes team Nemesis to use the Embody system which overpowers her choices once again. However, Aila's wish to quit the fight cause the Arista on her necklace, as well as the one in Reiji's bracelet and, unknown to them, the one in the Chairman's pocket watch, to resonate. With the unintended side effect of expanding the Plavsky Particles in the Arena Generator.
This phenomena causes the two fighters to land in a sort of parallel space or mental link, where Aila admits to Reiji that she fights only to avoid returning to the streets. Reiji tells her he too is a drifter, and that if she needs a place to go to then Sei would offer her a spot in his house. This leads Aila to quit team Nemesis (by pushing her sponsor out of the battle station and verbally attacking the chairman of team Nemesis and his grandson) and ditch the Embody system. However, she still fights Reiji after the two broke into an argument before the astonished crowd (plus Sei).
Aila is able to watch the Plavsky Particles in the battle system and predict the movements of her opponents, which ensure that she avoids attacks. However, the Star Build Strike RG System's ability to charge particles both inside and outside the Gunpla overdrives her senses, preventing her from avoiding Reiji's Knuckle Attack. After the battle, the two fighters share a drink in the stadium drinks machine area and Aila reminds Reiji of his promise to have Sei offer her a place to stay, to which Sei immediatelly complains to Reiji, but ultimately agreeds to.
During the fight between Tatsuya and Julian, Reiji reveals a shocked expression at both F91 Imagine's ability to "split" into three units and the Amazing Exia's ability to use Trans-Am. Later during the Gunpla festival, Reiji uses a repaired Beginning Gundam against Sei's Build Gundam Mk-II, expressing his joy of how he can never quit Gunpla Battle while showing amazement at Sei's improvement in Gunpla piloting.
During the final match against Master Kawaguchi, he notices that there is something off about him. Reiji pilots the Star Build Strike against Kawaguchi's Exia Dark Matter. The Star Build Strike uses most of it's discharge output against the Exia, only for the beam to be dispelled by the latter using an ability that Sengoku Astray possessed. As the fight rages on, the Star Build Strike is damaged beyond normal repair, having its support booster:The Universal Booster destroyed, having its beam sabers nullified, and having its vulcans torn off. The Star Build Strike then is de-activated during the match.
Reiji, in disbelief, hears from Aila through the Arista gem on his bracelet that Tatsuya is being controlled. Finding a purpose to fight on alongside Sei, Reiji re-activates the Star Build Strike, using it's left hand to slam it against the wall to propel it forward while Sei purged the right arm of the Star Build Strike. Using one of the blades (from the Exia Dark Matter) that was impaled in the arm, Reiji gains the upperhand as Mashita, growing in fear, orders Tatsuya to retreat and have Miss Baker change the field.
Chasing the Exia Dark Matter in the Space Field, Reiji finds himself in trouble as the Exia purges his Dark Matter booster and uses it as an independent fighter against the Star Build Strike. As the Dark Matter booster approaches the Star Build Strike, Reiji uses the remaining 10% of discharge on his right leg to create a circle of plavsky arrowhead daggers to encase the Dark Matter Booster, disrupting its particle generation, and slowing it down. Reiji then uses the remaining 9% of discharge on his left leg to create two small power gates to shoot the stolen Brinicle Blade as a projectile, destroying the Dark Matter booster.
As the booster is destroyed, the GN Drive re-docks with the Exia as they have one final showdown with the Exia chasing the Strike down with its GN Vulcans. Sei tells Reiji about the floating right arm in space and is able to place it back onto the Star Build Strike. Reiji and Sei use more plavsky power gates to empower the Build Knuckle, and launches a double fisted attack that they dub as Double Build Knuckle and charges straight toward the Exia Dark Matter, wielding two beam sabers. Exia in the resulting fight is damaged as the dark red painting scheme chips off, revealing the Amazing Exia, and freeing Tatsuya from the Mashita's mind control.
In the aftermath of the round, the arena is transformed into A Baoa Qu by a rampant large Arista crystal. Alongside others, Reiji uses the damaged Star Build Strike and charges toward the gigantic space fortress. He and the others encounter Mocks, new Gunpla developed by PPSE and subsequently steals one of their ranged weapons and uses it for the remainder of the battle. As reinforcements come in, Reiji, along with Sei, Aila, and China head into the center of the fortress only to realize that there is a massive particle cannon stored there, destroying dozens of Mocks, along with China's Bearguy III and Aila's Miss Sazabi. As Tatsuya and Takeshi enter the battlefield to assist Sei and Reiji, they head to the room with the giant crystal. Sei undocks his Build Mk-II Booster and attaches it to the Star Build Strike, Reiji then uses the Hyper Build Knuckle, destroying both of its arms in the process.
As time is limited because the giant Arista crystal was the source for all Gunpla battles, Reiji uses the hastily repaired Star Build Strike that has pieces from the Gundam Fencie Rinascita and Crossbone Maoh in a fight with the Amazing Exia Repair. He tells Sei that what he lacks isn't the skills to pilot a Gunpla, but rather the resolve to move forward even after getting his Gunpla damaged. As Sei takes control of the Star Build Strike Repair, Reiji's Arista finally gives out - causing him to fade back into Arian. Reiji believes that he'll be back because Sei and him are always connected and wants to battle Sei when he gets stronger.
To Reiji's shock at his return to Arian, he finds Aila present there, as she had made a wish to be with Reiji (and also possessing an Arista gem). However, Reiji is dense, not realizing Aila's feelings for him which causes her to lash out at him.
- His name is a reference to Mobile Suit Gundam École du Ciel and its protagonist, Asuna Elmarit. Aria and Ciel can both mean "Sky".
- While he is far more emotional, his attitude, lack of tact and human socialization, and preference of confrontation (such as barging into the CEO of PPSE after beating up all his guards) seems to resemble the young soldier archetype as exemplified by Setsuna F. Seiei and Heero Yuy, since both tend to give answers that is too straight forward, and prefer to "solve" problems through fighting.
- While piloting, Reiji seems to take excitement in fighting other opponents and beating them similar to Hallelujah Haptism though in a different manner where the super soldier slowly tortured and mocked his opponents until they were killed, Reiji just destroyed their mobile suits and shows signs of respect to strong fighters.
- Due to his skill as a pilot, Reiji is often confused to be a Newtype but it is unconfirmed by anyone. He also seems to bear some similar piloting skills to main characters from other Gundam series like Amuro Ray, Setsuna F. Seiei, and Kira Yamato, who at the beginning relied more on offense instead of defense.
- The line, "Our Gunpla isn't just for show", said by Reiji and Sei in episode 7 is a slightly changed version of Amuro's line, "This Gundam" or (from Char's Counterattack) "This ν Gundam isn't just for show".
- Reiji's romance with Aila pays some homages to Shiro Amada and Aina. Both couples are hostile to each other when they first meet, and eventually wind up fighting in mobile suit combat. The two couples are also isolated from the rest of the main cast, in Shiro and Aina's case when they disappear to live in a cabin in the woods, and in Reiji's and Aila's case when they both are teleported to Arian. Also, Reiji and Shiro pilot white Gundams, while Aina and Aila both pilot Zeon Mobile Suits of greenish hues (even her SD kit has mostly green parts). like Shiro and Aina , the two are married and have a child. Aina and Reiji are the scions of rich or well-placed families, while Shiro and Aila are fighting because they must/choose to. Aina also pilots Mobile Suits almost entirely from the Universal Century.
- Reiji's only self-built Gunpla is ironically from the very series that Build Fighters is based on, and was the protagonist suit in that series. Despite this, it only saw a few battles, (even less than the Build Gundam Mk-II) and was only used when Reiji couldn't pilot the Build Strike Gundam.
- Tatsuya Yuuki wears sunglasses similar to Reiji's in Gundam Build Fighters Try.
- According to Episode 15, his stats are of Follow:
- Attack accuracy: 94%
- Attack evasion: 57%
- Attack blocking: 81%
- Ground battle rating: A-
- Aerial/Space battle rating: A+
- While Mashita does recognize Reiji as a prince from Arian, the latter's inability to recognize him is likely due to several factors:
- Mashita had long disappeared from Arian, making his image faded from public.
- As Reiji is 14 during his arrival to Earth, he was 7 by the time Mashita is still on Earth and children below 10 years old are likely to have low memory skills.
- Sekai Kamiki strongly resembles Reiji both in appearance and personality.
- ↑ Gundam Build Fighters Episode 1, Sei and Reiji
- ↑ Gundam Build Fighters Episode 2, The Crimson Comet