Disappointing

Idk if this will help since this is my personal speculation but...
(Spoiler )
I think that the guy who just stabbed the black haired guy ( was it Kang, idk) is the older version of the red haired guy/cuckoo (which would make sense considering how there are diff universes?) and the reason why they are all obsessed with Yuna is because the one who got them all together is also the one who is obsessed with Yuna/cuckoo/the older version od cuckoo from a different universe. This is because cuckoo's parents died and Yuna was there for him (kinda. They stayed together in the orphanage) But she left him because her new father (the white haired man with the scar) picked her up. That's what makes her special for cuckoo because since then he is obsessed with her. We know that the blonde guy who has amnesia came from some sort of different parallel universe and all he knows is that he wants to protect Yuna from something. He heavily relies on that so he prob has some sort of deep connection with Yuna, but we don't know yet. I agree that Yuna isn't really special in the general aspect, but she sure is for cuckoo.
The reason why the other people involved in the group are doing what they told is because of personal circumstances. Cuckoo is obsessed with Yuna anyway, the twins were involved in a murder and had to keep going on, the class president also indirectly killed someone. The only difference between him and the twins us that he does his best to get away from the group/stop being involved with them since he really was just forced into being part of it.
I hope this helped!（＾ｖ＾）