This is the belief of a 'close' relationship between friends Akira Amatsume and Kazuha Migiwa. Though at first Haruka mistakes them for being lovers given the sensual conversation between them was overheard coming from the girl's locker room, It is revealed by Kazuha to Haruka she and Akira are half-sisters who share the same father and while Kazuha is aware of this knowledge of her relation to Akira, Akira, however, is unaware of her and Kazuha being half-sisters and believing her overprotectiveness and concern for her is really done out of a romantic nature, Akira kisses her and Kazuha steps back shocked by this sudden move by Akira and runs away emotional and sobbing.
- In the manga novelization of the series, Akira holds a romantic interest in Kazuha due to her overprotectiveness before it's revealed that although they shared the same room in the hospital they were both born in, they weren't actually blood related due to DNA test results. In the series, however, this was adapted to both girls having a strong connection with one another (Akira's mistakenly being romantic) before it's revealed by Kazuha that they are actually half-sisters.
- This is the second character relationship in the series in which the characters share an unfortunate incestuous kiss due to them being really half-sisters (Akira didn't know; Kazuha knew and was appropriately shocked by this action), the second being between identical twins Haruka and Sora.