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|“||Even though my chances of winning are slim, I still can't run away.||”|
Yūma is shorter than the average for his age range. He has white hair and red eyes. It is later revealed that his hair was originally black and his body had stopped growing since he was 11 years old because it was made out of trion, thus explained his short stature.
He wears a blue suit top and gray bottoms traditional of Japanese high school students. When not in Trigger mode or school uniform, he usually wears a purple hoodie with a paw symbol and the text "UMA" (which stands for "Unidentified Mysterious Animal"), and beige baggy jeans pants.
In Black Trigger mode, Yūma gains a black jumpsuit which Replica fuses itself with. In C-Rank trigger mode, unlike most other trainees, he gains an all-black uniform, instead of the classical all-white one; it has a black jacket with thick white stripes running down from the sides of his collar to the sides of his sleeves and also around his collar and thin gray stripes in the borders of the zip fastener and collar.
In B-Rank trigger mode, he has a uniform similar to Osamu's, except his jacket's left sleeve doesn't have a shoulder guard. His right arm also has the same design as his left, unlike Osamu's, making his jacket more symmetrical. Also, the black stripes on his jacket's sleeves are thinner and, unlike Osamu's, don't have white stripes in the middle, and the black stripes go over his shoulders and collar. On the sides of his shoulders, he has Tamakoma Branch emblem, surrounded by the stripes. In addition, the sleeves of his jacket somewhat merge with his gloves, and he tucks his pants inside his boots, like when he was a C-Rank agent. His Unit's emblem is located on his left chest.
Yūma has a laid back personality and is not very knowledgeable about Japanese customs, being from another world (despite having a Japanese father), leading to some run-ins with rough people. He is logical, advising Mikumo to stay away from the trion Battle Soldier rather than save the students as Mikumo could die. He seems to be aligned with Border's cause, fighting against Neighbor forces. Despite his happy exterior, he is actually deeply depressed due to his father's sacrifice to revive him.
Initially, his only sense of purpose was to revive his father, as he felt tremendous responsibility for his death. However, after meeting Osamu, Yūma decides that his purpose is to support Osamu in his goal.
Yūgo is Yūma's father. Yugo tries to keep Yūma out of danger by discouraging his participation in serious battles, but Yūma does not take him seriously. As a father, Yūgo took time to teach Yūma life lessons. He loved Yūma deeply and died to save him, passing on his side effect and black trigger. Yūma wishes to bring his father back to life. He is baffled by why Yūgo sacrificed himself for a son who got himself killed because of his own disobedience.
Yūma seems to enjoy Replica's companionship and respects his opinion and becomes devastated when Replica is deactivated and taken away to Aftokrator.
Yūma and Osamu share a mutual respect for one another. Yūma admires Osamu for his honesty, sense of duty, and eagerness to help others at his own cost. Osamu looks up to Yūma for his fighting experience and strength and wants to give Yūma a reason to keep living. Yūma is considerate of Osamu's position in Border. He tries to advance Osamu within the organization's ranks and decides to take on the task of taking Osamu and Chika to A-Class.
Yūma is impressed by Chika's abundance of trion. Just like Osamu, Yūma is affectionate towards Chika and strives to make sure she is safe.
Yūma respectfully acknowledges Jin's strength, admitting that he might lose to Jin in a fight. However, he trusts that Jin would not become his enemy. Jin looks out for Yūma, directly facing off against HQ multiple times to stand up for him. When HQ sends the Expedition Force for Yūma, Jin refers to him as his "cute junior" and proceeds to fight for him.
Kirie singled Yūma out as the strongest of the Osamu-Chika-Yūma team and agreed to help train the team only if she got Yūma. She looks down on Yūma as "midget" while demanding his respect. They form a mild rivalry during training, and both see the other as a worthy opponent. Kirie seems competitively proud of her trainee, proclaiming Yūma's potential to the team when Chika was considered the best candidate first.
After Midorikawa defeated Osamu in Battle, Yūma stepped in, beating him after deliberately losing the first few rounds and completing the deal of Shun calling Osamu "Senpai." They quickly become friendly rivals afterward. Midorikawa is also one of the few people to refer to Yūma by his given name instead of his surname.
Yūma seems to have a good relationship with Murakami, as they are seen having mock battles since their first battle, most of which ends with Murakami's victory, usually winning by 2 points every time they fight. Because of his Side Effect, and Murakami being the #4 Attacker in Border, Yūma seems to respect him.
Yūma and Kageura seem to have a good relationship, despite their rocky start. During their match, they assist each other in defeating Sumiharu Inukai and Shinnosuke Tsuji to save Chika, despite being on different teams, and they later schedule a ranked match, although it doesn't take place as Yūma gets lost on the way there, and is eventually "summoned" by Mikumo. Murakami stated that Yūma would appreciate Kageura's honest nature, and would be compatible with him.
Under her position in Border, Yūma seems to respect Kako, somewhat knowing of her skills. She wanted to recruit Yūma to her unit due to his skills, and to his surname starting with a "K," as she wants to form a team of pros with surnames starting with K. She even jokingly considered "recruiting him by force," to which Yūma said he'd fight back.
- (To various people) "You make up the stupidest lies."
- (Talking to thugs.) "I told you that I'd send you flying. You guys... Shouldn't lie." 
- (To Miwa) "Now then, why don't we have a little discussion?" 
- (To Usami) "Hmph. In other words, if I want to become a B-Rank, it'll be alright if I kick the C-Rank guys' asses." 
- (To Viza) "You know.... You make up some stupid lies." 
- (To Yūma) "Even though my chances of winning are slim, I still can't run away." 
- (To Rindō) "I'm not really upset, we all trained the same way. I won't be able to win all the time, but, fighting strong people is fun. The person who thought of ranked matches is smart, especially the part where the loser doesn't die." 
- (To Murakami) "I got permission from my leader. This time, I'll be playing with you.." 
- (referring to his father) There are three lessons he taught me when I
was six. Lesson one: Protect yourself, by yourself. Parents can't protect you forever. Work out or think hard and figure out a way to take care of things by yourself. Don't get close to things you can't take care of. Use your imagination to avoid danger. Lesson two: There's more than one answer. Things can be resolved in many ways. On the contrary, there's no answer in some cases. Don't get hung up with one way. And finally, lesson 3: Don't think your parents are always right.
- "Yūma" is a combination of two words meaning "to play" (遊?) and "real, genuine" (真?), together his name means "to play for real" or "playing real," which relates to his attitude as a member of Border. "Kuga" is a combination of "sky" (空?) and "leisure" (閑?).
- The creator, Daisuke Ashihara, mentioned that even without Yūma's postponed phase, he would have kept Yūma at the same height if it were not a part of the plot.
- He also stated that he himself does not think of Yūma as small, but incorporates the concept of characters referring to him as so to show that height does not matter—also seen as how Yūma lets it slide, instead of viewing it as an asset.
- Yūma has a habit of doing duckfaces, which have become a meme among the manga's fans.
- Yūma tends to scare people with his eyes.
- His "UMA" hoodie might be a reference to his name, as "UMA" (ユーマ) and "Yūma" (遊真, ゆうま) are homophonous in Japanese.
- Yūma apparently dislikes being called "senpai," as he tells Midorikawa to call him just "Yūma" when the latter calls him "Yūma-senpai."
- Ironically, he previously ordered Midorikawa to call Osamu "senpai" should he defeat him in battle.
- He likes Japanese food.
- He came second in the First Popularity Poll.
- Yūma does not need to sleep, due to his natural body being made of Trion as well.