when the main guy went over to the black haired guy's (jigu) apartment jigu texted his online character and when the main guy saw the message he told jigu it was from an online friend and then jigu put two and two together (dk if this helps im bad at explaining and i have no idea what their names are) its pretty clear in the chapters when he realises though because he asks the main guy "are you close with that person" (sum like that) and then gets mad when the main guy said not really
I think he found out during the mandelein incident!
Is Jigu the ML? If he is the it was when MC change his avatar's appearance to ML's apperance
*It was when MC changed his avatar's apperance similar to ML. There was a lot of "coincedence" prior to that too.
Yeah, that made him suspect a lot but then the avatar appearance thing is what really let him know. Then the message he sent while making cookies was him testing his theory so now he knows for sure.
Wait how the hell did jigu find out