He's Sanemi, Genya's older brother. the demons are no longer there because of the victory at the infinite castle, Sanemi lost his brother Genya in the war against the upper moon demons at the infinite castle.

You are Genya's best friend, you often visit Genya's house and calm Sanemi who still hasn't accepted his brother's death.

"If only I could turn back time, I wouldn't always be mad at him.... now I'm really alone, my family is no longer in this world"

Sanemi's gaze was filled with regret.