Aika Tsube, a 15-year-old freshman at Yōgetsu Private Academy, is the childhood best friend of Souji Mitsuka. She lives opposite him and treats him like a little brother, her overprotectiveness masking a massive, unacknowledged crush. This tsundere dynamic fuels frequent jealousy when Souji interacts with other girls, culminating in visible distress upon learning of his engagement.

Physically, Aika stands 157 cm tall with thigh-length dark blue hair styled in twin-tails secured by white ribbons and striking red eyes. Her civilian measurements are 72-56-80 cm, and she carries a significant complex about her flat chest, reacting violently when teased about it, especially by her teammate Twoearle. Volume 16 depicts her transformation into a childlike form resembling Tail Red's appearance.

Trained in martial arts by her grandfather, Aika possesses exceptional combat prowess, specializing in high-position attacks from low ground. She transforms into the twin-tailed warrior Tail Blue using the Tail Gear system, wielding the water-element Spear of Water, Wave Lance. Her signature attack is Execute Wave, and Volume 8 introduces her upgraded form, Eternal Chain, enhancing her armor and overall power.

Personality-wise, Aika serves as the group's "straight man," reacting with exasperation to absurd supernatural events and other characters' antics. She maintains a rocky but caring relationship with Twoearle, alternating between physical retaliation for teasing and moments of vulnerability, such as apologizing for her behavior or expressing a wish for a more endowed physique when seeking a new tailgate. Her development involves reconciling her feelings for Souji and gradually accepting her warrior role despite initial public dislike, which improved after Tail Yellow joined the team.

Her family includes an older sister, Renka Tsube, and the grandfather who shaped her fighting skills. Her surname means "haven, boundary," while her given name combines "love" and "fragrance."