Amemiya is Yuzu and Mei's post-homeroom teacher as well as Mei's ex fiance. He was a sensei at Aihara Academy, but was kicked out of the school after Yuzu revealed his sexual advances on Mei. He has become an unpopular character in the Citrus fandom after his mis treatment of Mei.
In chapter 1, he meets Yuzu whilst she is cleaning off her make-up. Yuzu's first impressions of him are that he is "such a hottie!".
Later on when she follows Amemiya-sensei determined to get his email address, she comes across a horrifying sight. She witnesses Amemiya forcing Mei up against a wall and kissing her, and notices Mei's tearful eyes. She then sprints away, alerting both Mei and Amemiya to her presence.
In chapter 2, she and Harumin accidentally come across a telephone conversation between him and his mistress.
Worried, she tells Mei as soon as she comes home, only to have Mei admit that she was aware. Attempting to be a better older sister, she announces to the whole student body in their morning assembly that Amemiya was extorting a student. He is then promptly excused from his job at the school.
Mei Aihara - Ex fiance, it's hinted that he randomly forced her to kiss him.
Unknown Mistress - Little to nothing is known about the woman/man he was cheating on Mei with.
Yuzu Aihara - His homeroom student. The one that called him out on his tricks.
Mei's Grandfather - His boss. Little is known about their interactions.