If I don't assume Woosub has an ability to be at the right place at the right time, then he's a badly-plotted character. The man is a darling though; kind, accepting, and he never manipulates Rosa. He and Rosa shouldn't have romance (unless there's a lot more buildup), but Woosub's purehearted friendship really highlights how Han Doryeong lacks consideration for Rosa and currently doesn't deserve her affection.