I'm assuming the neighbors were Hikari's family, so that means Hikari got cucked?
Btw, it's been a while so I'll say what I think/know of the three characters, just to see if I'm wrong on anything, I feel like rereading would only make me more confused, so I'd rather not.
Yuu, is the guy, he kinda went with Yami cause he's a horny teenager and wanted to forget about Hikari because of his inferiority complex, eventually he did fall in love with Yami tho. After Yami broke up with him he had no way to find her so he lost all contact with her, he didn't do anything dramatic to try to find her either as it's typical on romance stories, he reasonably accepted it was over and eventually moved on.
Then he decided to focus on his feelings for Hikari that never did necessarily disappear even if they were dimmed, then Yami showed up, they talked, she hit him, then kissed her and then she went to confess to Hikari, she not only rejected him but pushed him away when he tried to kiss her. This last bit is fun tho, kuddo can't get close to either without being attacked lol.
Yami, is the dark girl, has tons of issues at home, so she tried to drown her sorrows in sex with strangers, met Yuu and got fixated on him cause she disliked him being a good kid, fell in love with him either with time or during their first meeting. She felt guilty of her mom's issues and as such decided she didn't deserve to be happy and so broke up and cut all contact with Yuu, it's also very likely that she was insecure of her relationship with Yuu, probably in part because she noticed Yuu's fixation with Hikari, ultimately she seems to have some deeply rooted feelings of self deprecation.
Yami then decided to try and have a normal life and started attending school, was about to drop out again but met Hikari, became friends with her and eventually found out Hikari was in love with Yuu, I think it went from a concern, to denial and lastly realization. Wanted to support Hikari but eventually her "thirst" for Yuu won, not to mean it necessarily as lust but as simple dependence. Yami hid from him for a while, but as she wanted him to find her she came back to the classroom at the end of the day, and he found her while looking for Hikari. She hit him and then kissed him, and then... she went away I think? don't remember that part well, also I'm still not sure if she knew Hikari was looking, I think it's already been cleared up but I don't remember.
Hikari uh... Hikari is the protagonist, I've already said everything about her when describing the other two. I could perhaps add up that the first chapters of the story made us experience all of this in a really similar way to how Hikari might have experienced it, first we were really sure Yuu liked her, then he says he's already in love with someone else and we were sure he was talking about Hikari, then she decides to confess and everyone's excited about it, wanting to see what happens, expecting some sort of comedic attempt to delay the confession.
Then we see Yuu and Yami kissing, we don't really get anything, not even who kissed who, if they knew each other, how that happened. Since we don't understand anything we start backtracking, and all of those signs that he was in love with Hikari start to look like clues that he was in love with Yami and not Hikari, we start to notice things and overthink everything.
Then we get more of the story and seems like Yuu was actually in love with Hikari? we were already lost and now it's worse.
If anything I "like" that part of the story.