Aika Tsube, alias Tail Blue, attends Yōgetsu Private Academy as a 15-year-old student and shares a complex bond with childhood friend Sōji Mitsuka. Her overprotectiveness toward him, initially perceived as sibling-like, conceals unspoken romantic feelings that fuel a tsundere dynamic. This manifests in sharp jealousy when others approach him, especially Twoearle, whose teasing about Aika’s flat chest—a sensitive insecurity—often sparks physical retaliation.

Honed by her grandfather’s martial arts training, Aika wields agility and tactical precision to dismantle opponents, skills integral to her role as Tail Blue. Armed with the Spear of Water, Wave Lance, and later the Eternal Chain power-up (introduced in Volume 8), she defends Earth with enhanced armor and combat prowess. Initially disliked for her abrasive attitude, public perception shifts after Tail Yellow’s arrival softens team dynamics and earns her acceptance.

Aika’s growth centers on balancing personal insecurities with duty. A defining moment sees her apologizing to Twoearle for past hostility while hesitantly requesting a temporary voluptuous form, exposing vulnerability beneath her prickly demeanor. Often cast as the team’s “straight man,” she counters absurdity with exasperation yet steadily embraces her warrior identity.

Future Chain emerges in later novels, though its specifics remain undefined. Familial ties include an older sister, Renka Tsube, though their relationship lacks elaboration.

Her design pairs dark blue twin-tails tied with white ribbons, crimson eyes, and a 157 cm stature. As Tail Blue, her armor overlays a midriff-baring outfit, maintaining her civilian aesthetic while aligning with the series’ thematic fusion of twin-tails as visual and narrative symbols.