Shū Kuga has dark purple hair, typically styled with long bangs that partially obscure his purple eyes. He commonly wears his school uniform with an unbuttoned blazer, no vest, and a loosely knotted tie. For training, he prefers a black hoodie featuring white accents over a purple tank top, black pants, and dance shoes, often pushing the sleeves to three-quarter length.

Raised by his mother after his father's early passing, he observed her challenges supporting them alone. To bring her joy, he imitated musical performances from television despite initial lack of skill. Her positive response ignited his profound passion for musical theater, motivating him to pursue it professionally with the aspiration of performing for her in a genuine production. He assists with household chores when possible and reacts intensely against anyone distressing his mother.

Though initially perceived as aloof or solitary due to his quiet demeanor, he consistently shows concern for others. He maintains friendships, speaks decisively when needed, and supports peers in his subtle manner. Childhood friend Izumi Toraishi labels him a workaholic, though he displays occasional laziness and avoids unnecessary effort. He possesses poor facial recognition, frequently gets lost, and remains unaware of his own poor drawing skills. Pragmatic in habits, he states he will consume anything edible and expresses no beverage preference, considering tap water adequate.

His closest bond is with Izumi Toraishi, whom he met in childhood and attended middle school alongside. While Kuga formally addresses Toraishi by surname, Toraishi uses his given name "Shu." Their friendship permits Toraishi to borrow Kuga's possessions regularly, though items often go unreturned, creating tension. Kuga introduced Toraishi to musicals, influencing his career path. Despite their closeness, they compete fiercely, especially when championing their respective teams. As roommates and classmates with Kaito Tsukigami, they interact frequently, with Kuga relaying messages or borrowing items like chargers or toiletries. Tsukigami finds this borrowing aggravating, particularly as it stems from Toraishi failing to return Kuga's belongings.

A pivotal moment occurs when Yuta Hoshitani and Toru Nayuki discover him playing piano during lunch. Initially refusing their dance invitation to avoid soggy noodles, he accepts Nayuki's rice balls upon learning his own meal is insufficient. This encounter initiates his gradual integration with Team Otori. Later, he encourages a disheartened Hoshitani by sharing his personal motivation for performing and affirming that anxiety is normal.

He works part-time at establishments with pianos to merge employment with practice. A motorcycle serves as his primary transport, though its breakdown once caused him to miss Ayanagi Academy's entrance ceremony. He is self-taught in piano. Franchise "what-if" scenarios depict him as a florist, martial artist, or mafia member.

His musical contributions include solos like "RADIANT MIND" and "Seishun COUNTDOWN," duets such as "HEROISM++" with Toraishi and "By Your Side" with Tsukigami, and group performances including "Hanamichi ∞ Go All Out!!" and "Gift ～Curtain Call～" across anime seasons and OVAs.