Kamagiri Nobutatsu

Nobutatsu is the father of Kouta and is the opposite of his son. He is handsome and had success with girls in his high school days. He have a good relationship with Saya, the childhood friend of his son. He knows her since ever and seems to likes her like a daugther.
He works until late in the night and Kouta sees him only for the dinner and is very pleased to meet and eat with a family-like again.

Spoiler

He as an affair with Kouta's childhood friend Saya.

The reason he doesn't reveal his relationship with Saya to his son remains a mystery.