Chang Gai Steiner, twin brother to Lu Steiner and a second-year Seisho Academy student, balances athletic prowess and academic brilliance as the composed leader of the Shibuya Defense Service. His strategic calm starkly opposes Daisuke Dojima’s reckless heroism, a dynamic fueling his vocal disdain for what he deems childish impulsivity.

Bound by shared trauma, Chang Gai’s past intertwines with Daisuke, Lu, Marin Temari, and Keisaku Asano after their abduction by the enigmatic Milo during childhood. The incident left him with the mantra “Doubt everything,” a creed he upholds alongside his duties. When the group is hurled into a desolate future, his leadership fractures under apocalyptic pressures, clashing violently with Daisuke’s vengeful resolve.

This rivalry culminates in Daisuke’s shotgun ambush, a fatal act of retribution concealed from Lu, exposing the group’s unraveling trust. Physically distinct with caramel-blond hair, piercing blue eyes, and a towering frame accentuated by a crimson combat suit, Chang Gai stands as the defense team’s tallest male member. His guarded protectiveness toward Lu masks a nuanced bond—balancing her emotional dependence with his own stoic loyalty—a duality defining their fractured yet inseparable connection.