Well I was impatient af and decided to just read the raws, turns out the rival is her brother who turns out to be adopted...
Arata saw how affectionate he was with her and suspected something was up, he told he about it and of course she denied it and called it crazy because to her it was (she actually knew nothing at this point). Long story short they have a fight over it...
Now... the adoptive brother kissed the fmc when they were on a trip somewhere and got away with it because no one was near him at the time and the fmc was asleep. The second time was in the club room at school where the fmc was asleep and the brother did it again but this time Arata's childhood friend happened to be watching and took a picture(Im assuming it was with the hope of making Arata give up on the Fmc because she still had feelings for him.
Obviously she spread it online and soon after the entire school saw the picture...the fmc shocked by it rushes home searches the study room only find the adoption papers and coincidentally at that moment she has a flashback to when they were first introduced as kids.
So yeah this point all hell breaks lose
the brother tells Subaru that hes going to transfer when there at school then skip to when the go home there is a "fake attempted rape" scene where he basically pushes her onto the bed, kisses her neck then goes to kiss her lips but then stops.
Skip to school, Subaru faints because of stress and when she wakes up Arata has a convo with her about breaking up because she might love her brother more... they never give a clear answer and just kind of leave it in the air
Skip to the end of the day Subaru's friend tells her that she has to make a decision or else she will lose both her brother and Arata...
late at night the same day Subaru messages Arata and the agree to meet, but truck kun comes out of nowhere and unsuccessfully attempts to isekai Arata...he ends up in the hospital , I think he was unconscious for maybe two days.
Fun fact: the day he wakes up is Christmas
Right after Subaru and her brother visit Arata they finally have the long awaited talk...
Subaru literally puts his hand on her face and goes i quote
" This hand always makes me calm"
"It makes my heart Beat"
then suddenly goes from that to him being rejected
Skip to the final chapter
Arata moves into a new apartment (i think)
Subaru tells him she loves hima nd the makes out for a few seconds before the sore loser onii chan breaks it up...yada yada yada
Time skip again to when they pick up her brother at the airport because has finally finished school
NOW if you take nothing away from this at least read this part... at the end when they pick up her brother from the airport because he transferred, he obviously stills has feelings for the Mc, "he touches her hair and compliments her... he talks to Arata next and i fuck you not he says some BS along the lines of "The impossible is not always impossible" basically implying that he still hasnt given up on the Fmc and is going to try and ntr her from arata...............This is a load of garbage no one can change my mind