Hotaru only cares about one person in this world. That person is Yuma. The only thing Hotaru wants is to be with Yuma. Yuma finally confesses to her and this should of made her 100% completely happy but the mangaka had to throw in the overused trope where Hotaru ran away and left Yuma with no clue of where she went because now she doesn't trust Yuma even though they have spent their lives together. of course in the very last chapter Yuma goes and find her and even though there back together that havn't worked out their usses at all. nothing is really resolved and you walk away from this series not feeling good about their future. Very poorly written. This is probably popular is becuse the girls are cute. Other than that, this series doesn't have much going for it.

Hotaru's het relationship is crumbling from abuse, and Yuma's het relationship barely gets off the ground and crashes early due to lack of interest. So the infidelity really only lasts about five chapters. Frankly, the infidelity that does exist is more the fault of heteronormativity influencing Yuma's decision making more than anything, so take that how you will.

While these 2 females are cheating on their boyfriends with each other, feelings are involved. These girls are in love with each other. Neither of them were "stolen" away or NTR'd. They fell for each other. Surprising for it to be as dark as it was, the ending is pretty happy. Yuma and Hotaru end up together.

Yuma was the only one Hotaru was able to be friends with, so she was always with her and she ended up falling in love with Yuma. Our dumb protagonist never knew (but it hasn't been explicitly said that Hotaru loves her).
Yuma is able to make friends with other girls, unlike Hotaru who only has Yuma, so at one point Hotaru gets sort of left behind by her. She gets a boyfriend to fill the time she doesn't spend with Yuma anymore, which hurts Yuma for "some reason".

Hotaru begins kissing, groping and pleasuring Yuma, who never really says no or makes some resistance except for one occassion where she does reject Hotaru. The Fujiwara sees them kissing and learns their secret and from here things go downhill.

In the end Yuma gets depressed after fighting with Hotaru and breaking up with Takeda, which Hotaru realizes. Hotaru then goes to Takeda to ask him to try it once more with Yuma and gives him a talk in which she indirectly says she loves Yuma, but because she is hurting she will give up on her.

Things get brighter for the dumb girl, but Hotaru distances herself from Yuma who is oblivious as to why. Fujiwara gets his hands on blackmail material of Hotaru and Yuma on a steamy situation and as of now he hasn't used it. Hotaru begins working at a cabaret so that she is able to move out from her house to get away from Yuma, her neighbor. Yuma finds out and gets depressed.

TL;DR: Hotaru loves Yuma, but Yuma doesn't know. Yuma loves Hotaru, but she is dumb enough to not realize it. Fujiwara is an asshole and Takeda is a kind soul in the middle of three individuals who are going to hell.

Hotaru and Yuma are childhood friends who live next door to each other. They've always been close. When they grew a little older, Hotaru started dating one boy after another, presumably sleeping with them, which isolated her from the rest of the girls at school. Yuma is more innocent in that regard, and just got her first boyfriend, which made Hotaru pretty jealous. Under the pretense of "practice ", she started kissing and touching Yuma. Yuma isn't sure what to do. On the one hand, she likes her boyfriend, but on the other hand, she enjoys Hotaru's intimacy more.