Rui Kamishiro is a university proffesor of higher mathematics. To everyone, he seems like a young, responsible and single teacher, but in fact, he has been secretly dating one of the students, Tsukasa, for a long time. They obviously hide it, but when no one sees, they allow small tenderness. Kamishiro, despite his relationship with Tenma, remains a serious teacher. He lives with Tsukasa in the same apartment and sometimes he can help with upcoming exams. Tsukasa is 21 years old