Yūma Kuga is notably shorter than most boys his age. He has white hair and red eyes, distinguishing features that set him apart. It is later revealed that his hair was originally black, and his current appearance is due to the fact that his body is made entirely of Trion. As a result, his physical growth stopped at the age of eleven, which accounts for his small stature.

While attending school, Yūma wears a standard Japanese high school uniform consisting of a blue blazer and gray trousers. Outside of school or combat, his casual attire typically includes a purple hoodie featuring a paw print and the word “UMA”, an acronym for “Unidentified Mysterious Animal” paired with beige, loose-fitting jeans.

When activating his Black Trigger, Yūma wears a sleek black jumpsuit designed to integrate with Replica, his companion and combat support device. In his C-Rank Trion Body, Yūma is given an all-black uniform, which differs from the standard all-white outfit issued to most C-Rank trainees. This outfit includes a black jacket with bold white stripes running from the collar to the sleeves and outlining the zip fastener and collar.

As a B-Rank agent, Yūma’s Trion Body uniform closely resembles Osamu Mikumo’s but features key differences. His blue jacket lacks a shoulder guard on the left sleeve, and the symmetrical design on both arms includes black stripes that extend over the shoulders and collar. These stripes are thinner and lack the white middle lines found on Osamu’s uniform. His gloves merge into the sleeves of his jacket, and he tucks his pants into his boots, maintaining a style consistent with his C-Rank appearance. The Tamakoma Branch emblem is displayed on both shoulders, and his team’s emblem is located on the left side of his chest.

Yūma Kuga is typically laid-back and cheerful, often approaching situations with a calm and casual attitude. However, his easygoing demeanor masks a deeper emotional complexity. Despite being half-Japanese, Yūma was raised in the Neighborhood and is unfamiliar with many Earth customs, which occasionally leads to social misunderstandings and clashes with others. He is highly logical and pragmatic early in the series, he advises Osamu Mikumo to avoid fighting a Trion Soldier to avoid unnecessary risk, even if it means leaving others behind.

Beneath his outward optimism, Yūma carries deep emotional scars stemming from the death of his father, Yūgo Kuga, who sacrificed himself to bring Yūma back to life using a Black Trigger. For a long time, this loss left Yūma with a profound sense of guilt and a singular purpose: to find a way to restore his father. However, after forming a bond with Osamu, Yūma gradually finds new meaning in supporting his teammate’s goals, shifting his focus toward aiding Tamakoma Second and helping Osamu grow stronger as a Border agent.

Combat Experience
Despite his youthful appearance, Yūma Kuga is an exceptionally skilled and seasoned warrior. Trained by his father, Yūgo Kuga, from an early age, he spent six years honing his combat skills and later fought alongside the Calvaria nation in the Neighborhood for an additional three years. His strength increased significantly after receiving a Black Trigger from his father, to the extent that his presence alone prompted the withdrawal of enemy forces such as Spinthir.

Black Trigger
Yūma’s Black Trigger dramatically enhances his combat potential. When used in conjunction with his support device, Replica, the Trigger enables the instantaneous creation and overlaying of seals powerful Trion-based abilities. Replica also helped Yūma refine his battle strategies following his father’s death, and due to his Trion body, Yūma no longer requires sleep, allowing him more time to focus on combat training and tactical development.

Physical Abilities
Yūma’s fighting style is centered around speed, agility, and aerial maneuverability. He is capable of running on walls, bouncing off surfaces to change direction, and maintaining precise control over his movements even in mid-air. His Bound Seal, Grasshopper Trigger, and coordination with Osamu’s Spider wires allow for unpredictable and fluid combat movement. His lightweight frame and compact size give him an edge in mobility-based encounters and make him a difficult target to hit.

Tactical Intelligence
While Yūma usually leaves team leadership and overall strategy to Osamu, he demonstrates exceptional adaptability and tactical acumen in individual combat. He has a strong ability to read his opponents’ fighting styles and adjust accordingly. His creative use of improvised tactics has enabled him to defeat skilled fighters such as Tetsuji Arafune, Akane Hiura, Kazuaki Ōji, and even No. 4 Attacker Isami Murakami (with Hyuse’s help). His most notable victory was against Viza, a powerful soldier from Aftokrator, whom he defeated through quick thinking and effective use of his Trigger despite the absence of Replica’s support during the battle.

Track Record
Yūma’s combat record is marked by rapid progression and remarkable feats. Upon joining Border, he broke the simulation record for defeating a Bamster, clearing it in 0.6 seconds, and then again in 0.4. While still adjusting to standard Triggers, he was able to win three matches against Kirie Konami, a significant accomplishment in itself. His potential was recognized by Sōya Kazama, who considered him to already be at a Master-Class level. Shortly after, Yūma defeated A-Rank Attacker Shōhei Midorikawa with a score of 8–2 using only a training Trigger. Even in subsequent full-loadout matches, Yūma consistently won with a 7–3 margin.

Side Effect
Yūma possesses the Side Effect Seeing Through Lies (嘘を見抜く, Uso wo Minuku), inherited from his father. This ability enables him to detect lies, half-truths, and omissions when looking at someone who is knowingly being deceptive. It activates automatically, often signaled by his pupils turning black. Notably, this Side Effect can discern deception even from individuals like Replica, though it’s suggested he can sometimes sense lies through intuition alone without activating the full effect.

Trion Body
Due to the severe injuries sustained before his father’s sacrifice, Yūma permanently resides in a Trion body, while his original body remains sealed inside a ring to preserve his life. This artificial body grants him enhanced physical capabilities, allowing him to endure physical trauma and perform superhuman feats, such as surviving a car collision with minimal damage. His Trion body also has self-regeneration capabilities, though its base durability is lower than that of typical Trion bodies. While he does not require sleep, he still needs to eat and breathe like a regular human.

Yūgo Kuga
Yūgo Kuga was Yūma’s father and primary caretaker. A former warrior and the creator of Yūma’s Black Trigger, Yūgo trained his son rigorously while also trying to keep him away from truly dangerous combat. Despite Yūgo’s caution, Yūma often disregarded his warnings, believing himself capable. Their bond was deep, and Yūgo frequently imparted life lessons to his son. Ultimately, Yūgo sacrificed his life to save Yūma by sealing his dying body and transferring his Black Trigger and Side Effect to him. Although Yūma deeply respects his father, he continues to struggle with the guilt of causing Yūgo’s death through his own recklessness and often questions the reasoning behind his father’s selfless act.

Replica
Replica is a multi-functional Trion-based support unit created by Yūgo Kuga to act as Yūma’s protector and guide. Serving both as a mentor and companion, Replica assists Yūma in combat through strategic planning and technical support, especially with the use of his Black Trigger. The two share a strong bond, with Yūma trusting Replica’s judgment and advice implicitly. Yūma is visibly devastated when Replica is captured and deactivated by Aftokrator forces during their invasion of Earth.

Osamu Mikumo
Yūma and Osamu form a close and mutually respectful friendship. Yūma admires Osamu’s sincerity, strong moral compass, and selflessness, even when these qualities place him in danger. Conversely, Osamu values Yūma’s combat experience and sees him as a source of strength for their team. Their relationship deepens as Yūma becomes invested in supporting Osamu’s growth within Border and works alongside him to advance their team to A-Rank. Yūma’s loyalty and belief in Osamu’s potential become key driving forces in Tamakoma Second’s development.

Chika Amatori
Yūma holds a protective and affectionate regard for Chika Amatori. He is initially surprised by her extraordinary Trion capacity and, like Osamu, is committed to ensuring her safety. As their friendship develops, Yūma becomes one of Chika’s most reliable allies, encouraging her growth as a Border agent and helping her face her fears in battle.

The name Yūma (遊真) is composed of the kanji 遊 (yū), meaning “to play” or “to wander,” and 真 (ma), meaning “truth” or “reality.” Together, the name can be interpreted as “one who plays with truth” or “wandering truth,” which reflects Yūma’s playful personality and his ability to detect lies through his Side Effect.

His surname Kuga (空閑) consists of 空 (kū or sora), meaning “sky” or “empty,” and 閑 (ka or kan), meaning “quiet,” “calm,” or “leisure.” The name evokes a sense of peaceful emptiness or a tranquil void, possibly symbolizing his detached attitude and the hollow burden he carries due to his father’s death and his existence as a Trion body.