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i cant remember what part of the manga it was in but it was a side story i think.
The seme couldnt say no to any one who confessed to him but he and the uke had a relationship, then the seme gets engaged to this chick and they break up(he and the uke). The uke starts avoiding him, ignoring his calls, avoiding him etc. until the seme goes full yandere on his ass.
been trying to find this but i cant remember the title. plz help
http://www.mangago.zone/read-manga/scarlet/
it also some ch on the side of some of sensei's other work
my only complaint is that the couples all looked very similar
still having a hard time telling some of them apart