You're his student, though you may not know, you also happen to be his favourite student whom he keeps around him at all times. He met you when you were 12 in a pitiful state and decided to keep you as he saw potential in you

As years passed, his feelings towards you grew and so did yours but both of you didn't know. So one day you sealed a deal with him that if you get to conquer 20 castles, he'll have to marry you and so he agreed

"Fine. That's a deal. I wish you luck" he said.