Mimi is a young girl that
Tsukumo crossed a night in the street during his jogging, she searched her losting white cat nammed Sugar. Some times later, he learned she's one of his kouhai in his school.
She's the best friend and classmate of
Akeno. She tends to faint after a strong emotion. She is also a very pure girl.
Since Tsukumo saved her, she talks about him as her "Ouji-savior" - because she didn't know his first name - to her best friend, Akeno, who made the connection with Tsukumo when she asked her if she knew any something else about him and she replied that she only knew his last name, Sakurazuka.
(spoiler hidden)