If y'all want spoilers....
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Basically the friend is in love/lust with the MC and the white haired demon dude uses MC's form to get the friend to fuck him multiple times (and it looks like this will continue). The friend then increasingly spirals into this weird sadistic, fucked up part of himself and he basically becomes kinda twisted. I think it's because the demon dude put something in him that causes evil shit (prob whatever in his eye this chapter). Also though, I can't read Korean for shit so take this with a grain of salt.
On a completely unnecessary note of my opinion: kinda fucked up but I want to see white haired demon become infatuated with the friend andddd they end up together lmao. B/c white haired demon looks low-key tsundere? yandere even loool.
Y'all can see the raws in here https://toonkor.tube/디어-도어-DEAR-DOOR

i almost threw up reading the raws (what happened to the MC friend) it's disgusting, i think i'm going to drop this or just skip the parts with sid and mc friend