Responses
YEAH, I AGREE. I think it's that person that meet Seongin asking for lighter or what I forgot, it's him! This is my theory, but I think it's him who pushed Minwoo because when Seongin ask Seunghyun if he went out that night(the same night that Minwoo got pushed), Seunghyun really really seemed so clueless...and I think that he really didn't go out that night. It was someone else.

MC is blackmailed by his roommate. Roommate comes from a group home, also known as, foster care, aka government housing for minors without guardians / parents / living relatives.
Roommate is a twisted sunovabitch and enjoys seeing MC in psychological pain. This manifests as the roommate isolating MC from friends either directly, or indirectly.
MC, although he can tell what roommate is doing (in isolating him) goes with the flow because he is deeply ashamed of being taken advantage of, and thinks his friends are better off / safer when they’re not associated with him.
MC’s friend with benefits, Minwoo (the dude who ends up in London cuz he got pushed off the bridge), SUSPECTS that MC is getting blackmailed. He tells their mutual friend, Seonju. (The dude who punches MC in cafeteria while eating with roommate). Roommate finds out that Minwoo suspects.
The author wants us to think that Minwoo got pushed off the bridge by roommate. Minwoo launches investigation on who pushed him, and SUPPOSEDLY another passing pedestrian saw and testified against roommate.
I, however, DO NOT THINK SO. I think that the dude who borrowed the lighter from MC is a jealous bottom from roommate’s group home that got upset that roommate was spending more time with MC than him. So when he saw an opportunity to implicate roommate in trouble, he takes it. Think, “if I can’t have him, no one can.”
The author ended the season with Roommate in jail, and all of MC’s friends and family understanding that he was a victim and being blackmailed.
Anyway. That’s the whole season one.
Thank you for reading everything.