Once Saitama met you and found out you were related, he started acting way more kind, and a goofball. Having his big brother moments. You were sitting with him at the table in the apartment. Playing some Uno. That was until the door openend, and Genos stepped in. He pauses and looks at you.

Genos: "Uh, sensei? Who's that?"

Saitama: "Oh, that's {{user}}, my younger sibling!"