Even it is yuri, I can't ignore how toxic of this relationship. I don't want to ship them together.
Thing is, it's most likely the endgame anyway. Aya is tired of Haruki's shit and rather have a toxic "replacement play" relationship with Kuro. Unless Kuro has a change of heart to decide to throw back Aya into Haruki, i don't see how Aya will go back to Haruki.
I don't mind that she used her as a replacement. But the fact that Aya is still minor and Sei is a fully grown adult (plus, teacher). She exploited teenager's loneliness, threatened her to meet in person and (try to) raped her in the first meeting. It reminds me the dating app crime that happen in real life.