Yūma and Osamu share a mutual respect for one another. Yūma seems to enjoy Replica's companionship and respects his opinion and becomes devastated when Replica is deactivated and taken away to Aftokrator. It is a self-described "multi-task Trion Warrior" created by Yūma's father. Replica is Yūma Kuga's guardian and a recurring supporting character. He is baffled by why Yūgo sacrificed himself for a son who got himself killed because of his own disobedience. Yūma wishes to bring his father back to life. He loved Yūma deeply and died to save him, passing on his side effect and black trigger. As a father, Yūgo took time to teach Yūma life lessons. Yugo tries to keep Yūma out of danger by discouraging his participation in serious battles, but Yūma does not take him seriously. However, after meeting Osamu, Yūma decides that his purpose is to support Osamu in his goal. Initially, his only sense of purpose was to revive his father, as he felt tremendous responsibility for his death. Despite his happy exterior, he is actually deeply depressed due to his father's sacrifice to revive him. He seems to be aligned with Border's cause, fighting against Neighbor forces. He is logical, advising Mikumo to stay away from the trion Battle Soldier rather than save the students as Mikumo could die. 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. His Unit's emblem is located on his left chest. 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. On the sides of his shoulders, he has Tamakoma Branch emblem, surrounded by the stripes. 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. His right arm also has the same design as his left, unlike Osamu's, making his jacket more symmetrical. 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.
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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 Black Trigger mode, Yūma gains a black jumpsuit which Replica fuses itself with. 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. He wears a blue suit top and gray bottoms traditional of Japanese high school students. 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. Yūma is shorter than the average for his age range.