Hayami Riku♂

"Shall I teach you Katsuki-san? You're worried about the test right?"

The protagonist's homeroom teacher at the academy.
He's always calm and smiling and is popular with the students and other teachers.
But he really has an unconcerned attitude of 'what will be will be' about himself and others. He avoids interacting with others and his private life is cloaked in mystery.
He's lost his memories of his childhood, but since it doesn't affect his current life he doesn't pay it much mind.
Because he resembles her older brother Minato, the protagonist adores him.
After talking with him about her memories she was really interested to learn that he seems to have the same situation with losing his childhood memories as her.
After that he began to show her more of his true self and he started thinking of her like a little sister.
He currently lives alone in an apartment close to the academy.