He's probably confident that he'll beat Karam and Junseo when it comes to Jaewon cause he's the one Jaewon likes first.
As for him helping Karam, I'd say that's a backfire cause he was disappointed at the result in the end.
Yeah but, if he was confident, I still find him weird to back off when he's trying to confess then told Jaewon that it was "a joke" afterwards. I guess it's either he didnt like Jaewon 'seriously' or he's scared himself and waiting for the right momentum to confess. (and he's way too late for that)
Later on, he was trying to tell Jaewon that he was serious. He'd probably thought that passing it off as a joke would cause less fiction between them but it backfired. He's rather easy going compare to Jaewon after all.
He's weird but interesting wwww. Normally people would go hostile towards their rival (even Karam and Junseo dont like him because they like Jaewon), but he doesnt care about it, still as usual (being flirtatious) with Junseo, but keep saying that Jaewon is precious to him too.
I dont know if his "he's precious to me too." means that he likes him and wants to get him, or he likes him and wants to help him getting together with his love interest lol (I mean, even if I cant read korean, I still get an impression that Yoonsung is actually helping Karam, indirectly). but probably it was also unintentional, or his plan is just backfire...