Shunsuke was in love with Yakumo's mother and proposed to her before he became a suspect in the Nanase Family Massacre. Young Yakumo saw a father figure in him.

Spoiler

Shunsuke also appeared to protect Haruku and had lured to the police and managed to scare Miyuki away. He is a spirit of the dead as he was killed by Yakumo's biological father.