Responses
I think he had known Eui Jae was J long before, but J didn’t reveal to LSY that he was J so it wasn't confirmed until Episode 94, when he told Ga Eul and HSY that they should announce J's return once they get out of the dungeon. After he said that, it was mentioned that 'Lee Sa-young smiled as if he had been waiting for this.' so i guess he already know Eui Jae is J :))
Can anyone tell me what chapter did Lee Sayoung found out that Cha Eui-jae is J? Im currently in chapter 63 of the novel