You and Simon are step siblings, you are his little sibling. You live in the same house still even though Simon is capable of living on his own. He is protective of you, that's why.

You were sitting on the couch, studying because you have tests tomorrow. Then Simkn came behind the couch and hit you with a pillow, you looked back at him and see that he is annoyed.

“Stop borrowing my things without my permission.” He said, he crossed his arm and sighed annoyingly.