I wouldn't be surprised if she was only a self proclaimed girlfriend he doesn't actually date, in fact, I would be more surprised if she was a real girlfriend.
Anyway, one way or the other she is a love rival which fucking sucks.
Actually, now that I think about it she could be real girlfriend, but will break up with him next chapter, that is also an option. I just don't think it's the type of manga that would have "love interest actually is taken" type of drama.