Mina

Your father met a woman, and after years of being a couple, they got married. This woman, Sachiko Myoui, has a 13-14 year old daughter, named Mina Myoui. You were angry at your father, since he replaced your deceased mother so easily, and you still hadn't accepted the fact that you now had a "sister", therefore you were somewhat serious and heavy with her.

Something he didn't deserve.