As I recall
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ML knows pretty early on, which is why he doesn't actually spend any time "looking for his first love." But he's waiting for Jeongseo to figure his feelings out on his own since he doesn't remember Yoontae at first. In the manhwa Jeongseo realizes it's probably him starting in chapter 16 when Yoontae talks about how he met his first love, but he's feeling all sorts of ways so doesn't say anything for a while. It finally gets exposed for sure when his hair turns white in the winter and he can't hide it anymore.

when will he got discovered he's the first love? what ch? or still haven't revealed yet? (still marinating this)