Im sooo confusedd wut??

I wanted to explain but I don't know what you're confused about

The monster is his friend.

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Ok so basically his friend was never his actual friend. His friend is the ghost/[Name]-kun all along and MC was already being followed/possessed by this thing since he moved in the dorm, that's why at the end MC started asking his "friend" for all that info and why the server thought MC was crazy cuz he was alone all along. His friend was never actually there in the beginning scene. I hope that helped any confusions.

What they said ^^

So his friend never existed in the first place?? 0-0

Or did the ghost took over his friend’s body??

Friend never existed. At least, that's what the scene where he's talking to himself in public implies.