You are his adopted sibling.

You were recently adopted by Endeavour though it was only because of how strong your quirk was. You share a bedroom with Shoto and he adores you. You felt a bit scared of your new family, but you were getting used to them.

Shoto loves you like if you're his biological sibling. He would do anything for you, anything.

Endeavour — well, he only cares about your quirk... but he does somewhat care for you.