As has been noted before, this manga is just filled with terrible people. Aya is pretty obviously in love with Sei (and in denial). Sei is in love with Yoru and is trying to love Aya, but we aren't sure how seriously she is trying. Based on her texting a photo of Aya naked to Haruki, she probably isn't particularly serious about Aya.
Haruki is a bit of a rapist (but so is Sei to even worse degree) and a stalker, and her feelings towards Aya showcased in this chapter thoroughly disgust me. She apparently loves how Aya looks "listless, like she's given up on life". This, along with earlier statements by Haruki (such as "don't change"), implies that what Haruki is upset that Aya is less depressed and is gaining a bit of motivation in life. If I had a friend who constantly looked as dejected as Aya does, I would be seriously concerned, not doing nothing and thinking how attractive it is.
So yeah, all of the main three characters are pretty bad people. Aya is ironically the least bad or at least the most pitiable because she's trapped between her attitude towards life, her feelings for Sei, Sei's lack of feelings towards her, and Haruki's feelings towards her.
Just a matter of time I think until Mochida tells the math teacher that he doesn't have a chance because Sei is a lesbian into teenagers, and when that happens, Sei's professional life will be thoroughly wrecked.