Spolier!!

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From what I could guess from the Raw
Is the main character is just imagining the pink haired friend being there, cuz the pink haired one fell from a window when they were little, the main character think he was dead tricked himself in thinking that the pink hair guy was still around, but he wasn't so he would look like he was talking to himself when he thought he was talking to the pink hair guy, but the pink hair guy isn't actually dead he just left after the accident because the mom adventually gives the main character a phone number belonging to the real pink haired guy, for some reason no one told him that he moved away. I can't read Korean but this is what I got from the scenes and the way everyone was acting.
He ends up calling the number and meeting him at a coffee shop. He can still see the fake imaginery one walking behind the real one.