Lol... you literally just described one of the most classic examples of the power of friendship trope. She, the villain, will realize being alone sucks, and that she was misguided in her motivations all along. Even though she "killed" him, and continues trying to kill him, she'll be forgiven anyway. She then either becomes an "amazing fighter" (as you put it) that the protag can battle as a rival, or just joins his harem/team, and everyone is totally ok with it. Why? Because power of friendship. I'm sure Sera's sister will get the exact same treatment. Anyway, doesn't really matter, it ain't that deep.