Responses
Found it, lol. In case you guys are curious too, here's the link: (I really love the side couple—sunbae x delinquent guy troupe)
https://www.mangago.me/read-manga/my_damned_xxx/
IIRC, MC and ML are childhood friends. MC is in love with his sunbae that fucked him up (like played with his feelings). That same sunbae ended up with this blondish-haired guy that looks like a delinquent(?), sunbae also has a cat named Navi. I remember the details but can't remember the title PLEASE SEND HELP