Kei Higashide is a teenage student with distinctive waist-length violet hair and red eyes. She is the older cousin of Yuuta Yamauchi and tutors him during summer vacation as he prepares for university entrance exams. Kei possesses a notably forward and teasing personality, especially concerning Yuuta's romantic interest in a classmate, Youko Kaji, after finding a photo on his phone. This teasing deepens into a physical relationship as the summer progresses, shifting their dynamic from academic to intimate.

Her interests include playing chess and studying English, though these remain secondary to her interactions with Yuuta. Kei demonstrates protectiveness and commitment to their relationship when faced with external challenges. Later, after becoming a couple and living together, their relationship encounters opposition from Yuuta's sister, Otoha Yamauchi, who sets conditions for recognizing their partnership. Kei actively works with Yuuta to overcome these obstacles and secure Otoha's acceptance.

Kei's background centers on her familial ties and evolving relationship with Yuuta. Her development is marked by the transition from playful cousin to committed partner, navigating familial tensions to maintain their bond.