@Tcof: It could be this way, but from where I'm seeing it, I don't think it looks this way. Right of the bat she was butting into there relationship and is hell bend on finding fault. Each chapter she stops doing so, as she noticed, that it would be her, that would look like an evil witch, especially in her own eyes. As she finds most faults in other people, I believe she either has a tendential positive image of herself or, that something happened, that gave her a good reason to overthink her own image and now tries to convince herself, that she isn't an egoist (,which she fears, she might actual be).
Even though she surely enough is always back next chapter and tries to nag them apart.
Conclusion: She is envious:
Her relationship just blew up and it just happens, that her ex-partner just started a relationship. Suddenly she went from winner of the aftermath of their relationship to loser. Such a thing would be irking anyone, but in her case she is quite proactive to even the score out.
But there might be even more to that:
1. She moved back into the same city her ex resides in.
2. On their first meeting she came to the conclusion, Himari can do a pretty job on her own. It's like she tried to evaluate if Himari is a thread.
3.Afterwards she dropped the "I'm his ex-gf."-bomb, even after being told it would be tactless, just to shake her.
4. Everything above.
-> Theory: She
is trying to get back with the MC, even if she does everything to deny it in the story.