After rereading the few chapters that this Himari character is in, it seems like she's having trouble sorting out her emotions for Hitomi. She may not even be a lesbian by default, but instead is just attracted to strong people. Of course, Hitomi saving her from a couple of compromising situations heavily leans her to the yuri side, but her also being tsundere with her feelings doesn't help. Perhaps unintentionally, she's observing Hitomi and Yuu's relationship to gauge whether or not it's a romantic one. I can't say I'm rooting for her, but her presence would add a bit of spice to this rather predictable story.
