J rescued Sayoung in the pass, when he cupped his hands on Sayoung's face, Sayoung found it similar to when J rescued him from the past, that is how Sayoung realized that he is J
Sayoung was the kid drench in a purple poisonous stuff and J saved him . The kid is the only survivor of that outbreak, so he is special for J because J go beyond his way to save sayoung but j couldn't remember the kid's identity
Okay so Sayoung found out he’s J? Idgi was there something connected in the past