Hyakutarō Onigawara is a second-year high school student and member of Group K, participating in psychological challenges with his basketball club teammates. He displays a brash, impulsive, and easily provoked demeanor, often adopting a "tough guy" attitude, evident when he initially taunted Yuuichi Katagiri. Beneath this rudeness lies a deeply passionate and sensitive nature, particularly concerning past mistakes and fierce loyalty to friends. His defining experience involves the basketball club, where a poorly executed shot during practice seriously injured the team captain, Juuzou Kadokura. This incident fuels intense guilt, driving Hyakutarō to train relentlessly to support Juuzou's final shot at a national championship. His grandmother's advice to "be nice to females" shapes his behavior, resulting in uncharacteristic naivety and kindness around women, as seen with Maria Mizuse. Teammate Chisato Hashiratani identifies this contrast between his abrasive exterior and underlying concern as "tsundere." During the "Friendship Hide-and-Seek" game, Hyakutarō and Chisato are assigned to monitor food stations due to their speed, noted by Juuzou as the fastest in Group K. His straightforward movements make his actions predictable. Yuuichi Katagiri targets him as Group K's "nicest member," exploiting his protective instincts toward Maria. When Yuuichi pushes Maria off a slope, Hyakutarō abandons pursuit to save her, aiding Yuuichi's escape. Later, after Yuuichi feigns mistreating Maria, Hyakutarō defects to Yuuichi's group to protect her, despite Chisato's objections. This decision proves pivotal, as Group K's strategy relied on Hyakutarō triggering Yuuichi's disqualification through violence—a plan thwarted when Yuuichi restrained him. Physically, Hyakutarō has yellow/blonde hair and olive eyes. He struggles with mornings and prefers coffee loaded with milk and sugar. His name incorporates the characters for "demon/tile" (鬼瓦) and "hundred/thick/son" (百太郎).