I find it funny that people are losing it because of a situation that is not even a conflict. This literary solves the only potential conflict in the whole story - if Mizushima had feelings for Kimura, he could have been a rival. But since nothing suggest Hiyama has any feelings for him, all he can do is hinder the plot a bit. That assuming Hiyama isn't 100% gay, to begin with, which makes him a complete non-entity.
edit: I guess since Kimura is Kimura, there is also a possibility of the worst kind of conflict - the dumb one - where she'll be like "the boy I like is into that other girl, who is way better than me, so I'll just step aside and support them" or some shit like that... hate those.