My guess on their backstory

This is my guess for their backstory.
Seo-in and mc become friends and gradually develop feelings for each other. Seo-in being manipulative and closed off to his feelings came up with the “practice” for him and mc. That way he could test mc’s reaction without putting himself in a position to be rejected. Worse case, seo-in could play that off as a joke.

They don’t date and don’t confirm each others feelings but their relationship is progressing nicely. Mc is proper head over heels for ml. At some point the (we assume)- creepy step-brother maybe blackmails ml to kiss him or something and mc unfortunately witnessed that.

Feeling betrayed, used, and disgusted, mc tells Seo-in that their “friendship” is over and distant himself. (Which is a fact from chap 21) and Seo-in probably has no idea why mc did that and probably doesn’t know mc saw the kiss.

Which makes sense why he’s such a jerk in present times. Seo-in is in love with mc but thinks mc betrayed and abandoned him so even though he(seo-in) wants to date mc, he can’t reveal his true feelings for fear of it happening a second time. This is why he’s doing all this roundabout stuff. And, Mc doesn’t understand his actions bc he doesn’t think Seo-in has ever had any real feelings towards him.

I firmly believe he didn’t like someone else and that the kiss was forced bc 1. He’s soo obsessed with mc 2. The way he kissed mc (in the past) vs mystery man is clearly different. One is full of affection while the other is aggressive 3. Seo-in has been having flashbacks of their kiss from hs so obviously that was/is an important memory for him

Can’t wait to find out their backstory and see if I was right abt anything