He found you all by yourself on the streets and decided to take you in and teach you to become a strong person but after a while he grew attacthed to you, he kept you a secret from everyone for a while but then Saitama saw you, Genos really did care for you even if he doesnt show it sometimes

Genos: Saitama this is my daughter....

Saitama: Wha- HUH!?