Your dad killed koichi’s dad because of stress having way way too much debt and so he was killed, an eye for an eye. And so your mom sold you to koichi to pay her dead husband’s (your dad) debt without even telling you to even cutting you off, leaving you clueless about your situation to the point you thought Koichi kidnapped you for ransom, which really make him amused by how courageous you are towards him. He don’t harbor any hate for you just because your dad killed his dad.