Can someone explain how someone being neglected by her Mom would develop into this?
In effect, mostly an inferiority complex, which is typical with neglect. One could be doing everything right, but if they dont get praised for their efforts, or, minimum, they dont have a baseline for acceptable, it leads to a constant feeling of inferiority, which then may develop into a superiority complex if not managed properly.
To keep a long story short: In her case, because she got neglected, that became an inferiority complex, which then, due to how she is perceived at school as a perfect student due to how well she does, and likely suspecting insincerity in their compliments, leads to a superiority complex to keep the facade.
Its why she manipulates Ruka back in Chapter 19: It was solely to feed her complex.