" I may not be suited for a big bust sized Tail Gear, but it's not worth Soji losing their life over something like that . . . . gimme that blue bracelet !!! "
- Aika Tsube (津辺 愛香? Tsube Aika) / Tail Blue (テイルブルー? Teiru Burū)
- Both Aika and TailBlue are voiced by: Yūka Aisaka
- (Japanese); Sarah Wiedenheft (English)
One of the heroines of the Twin Tails, Aika is a 15-year-old freshman of Yōgetsu Private Academy, and a best friend of Sōji since childhood. She treats Sōji like a errant little brother, and is rather over protective of him, although this is later revealed to have occurred because of her massive romantic crush on him. Aika has complex dysphoria with the look and shape of her body, especially with her flat, underdeveloped chest. She has a violent attitude toward Twoearle, who often teases her about her small, childlike chest. If it wasn't for her long, blue Twin Tails, many humans and Elemelians would often mistake her for an adolescent boy. Aika almost always can 'get the better of' Soji, both mentally and physically, since she is quite intelligent, and has been trained in the Martial Arts by her Grandfather. She lives next door to Soji and Soji's mother, and likes to climb out of the window of her bedroom, leap across the roofs, and surprise Soji by entering his bedroom through the window.