I don't think 15 year old Seo-in is opportunist. Both of them are just normal kids at the time.
It's after Seo-in brother starts to bullying YW that things get messier...
15 Seo-in wasn't calculative or opportunist as he is an adult. Younger Seo-in looked bright and cheerful and adult Seo-in ... well... looks like hates everyone and doesn't care a shit about anything.
At this point it seems Seo-in has became so cold and cynical becayse of the trauma of having his first love disappearing from him... he resented YW so much but he never let go and has became obsessed and manipulative to the point to get YW as his manager.
He has been living on self destructive mode for awhile ... and YW wants to snaps him out of it.
But i do agree that the kiss may be initiated by the brother. He probably already hated YW, and we know he bullied YW after Seo-in went away for 3 months... He probably wanted to make Yw away from Seo-in.
Those are good points. Maybe opportunist is not the best word to use, but I do think it would track that if he had a slight curiosity about YW and things lined up with the student buddy program he would take that opportunity to check out YW. Again, this is all under the presumption of the trope of like, love interest meeting MC before their actual encounter.
I keep thinking about when they first met Seo said,” I’ve been a fan of yours”. After finishing season 1, I’m curious if there’s more to explore on this or was it just used for character build up. I say this because he handed YW the medal he threw after his match. Was he there as a spectator for friends, or was his step brother into judo? And that’s how he met him from the sidelines? BECAUSE…. Seo is an opportunist. We do see his capacity to be manipulative, but I think the “accidental” run in with the actor in Chp 1. gave him the opportunity to move things in his favor ( which, I think the actor is pretty manipulative because we know he set YW up to see him due to unrequited feelings). That being said, I’m curious if there was an opportunity to connect with YW through that mentor / buddy program.
If he met YW because he went to support a friend or his step bro at a match, then I think the dialogue during the jump flash back supports this when the kids were dogging him about his background in judo. He also was seen with a sling in the final chapter flash back so if that was a broken arm or dislocated something that was pretty personal (duh) because he needs his arm for holds. Like that wasn’t just a warning, that was definitely the step brother being like, “ get the fuck away from my man.”
I think the brother kiss flash back also showed that most likely the step brother initiated the kiss because he was gripping on Seo. Like, I think it’s been in the comments that it looked hella staged so YW WOULD see them. This then leads to Seo’s spiral once YW runs away. I think the scandal mentioned in the last chapter has a young Seo, I am willing to bet that pervy uncle had something to do with Seo being there. Like, him trying to comfort Seo by leading him straight into the lions den.
This is one of the only BLs I’ve read where I’ve been more interested in the what got us here than the present story. I was underwhelmed with the finale because the present timeline seemed predictable, like of course they’d get to this point of boning and at least one of them owning their feelings like it was some big discovery that the guy in high school he messed around with he had true feelings for.