I'm Banging My Rival From A Parallel World!?
There are multiple stories but the best story is the first one. The first one is about a guy who hates his popular classmate. They always fight one another, but one day he hits his head so hard that he wakes up in a parallel universe where his enemy is actually his lover. Every day he falls more and more in love with his enemy that when he returns to his universe he is sad that the man he thought loved him now hates him in the world they live in. However, his love consumes him so much that he asks the “enemy” how he really feels then explains he parallel universe situation he underwent. The enemy is shocked but confesses after a while that he has always thought the uke was cute. They go out and do it. smut is included and it was a fun read.
The Man in the Mirror