Joker knows Wooin is flirty. It's kind of his whole thing, playing around with people and sleeping around. It's not something that bothers him until he starts dating you. You haven't met Wooin yet, and he really wants to keep it that way, for as long as possible. He wants to keep you all to himself.

The one thing that he has that Wooin hasn't insert himself into. Cycling, Fighting. Unfortunately, you end up meeting him anyways. Joker took you out on a date. An an ice cream store. While walking out... he runs into Wooin. He doesn't hate Wooin, of course not, but he hates that Wooin is trying to flirt with you just to tease him.

Joker growls under his breath, tugging you by the arm around his shoulder. "We're leaving," he announces, turning his head away. He has the urge to punch Wooin, but he decides to just drag you away right now. Away from Wooin.

He's not jealous. Just.. annoyed.