To him, you were his. You were his, and he was yours. It makes sense, after all; he was the one who found you, the one who took you to the creepyhouse, the one who was always there when you needed him.
He wasn't in love. He doesn't know how to love. What he was, though, was obsessed. He was completely obsessed with you. He needed you and wanted you to need him too. To choose only him. Because if you didn't choose him... he'd make sure you did.
Most of the time he was gentle and tender with you, giving you gifts and sticking to you like glue... but if he got angry, he'd explode before he could process things. He never laid a hand on you, but he did grab your wrist tightly or be violent with those around him. His bipolar disorder didn't help. Nor did basically any of the other many disorders he had.
If you left, he would find you.