There are two ghosts that hunt Heesoo in the story: first is Heesoo's highschool classmate posing as the ghost of his lover, and the second one is the actual lover (Kim Taeyung)
The first ghost is Song Kitae, a loner who had a crush on Heesoo and was chronically delulu. He mistakenly thought Heesoo was in love with him just because Heesoo was the only one treating him kindly (it was actually their homeroom teacher's request for Heesoo because Kitae couldn't make any friends). He anonymously posted his story/obsession on a forum. At first, some people kinda cheered him on, but his behavior became more and more extreme, to the point of being a perverted stalker. Anyway, one day, he saw Heesoo and Taeyung kissing (Taeyung knew Kitae was watching but he just let him be without saying anything to Heesoo). After that, Kitae became resentful and started calling Heesoo a slut and stuff, and yet he still lusted over him and even began to steal his stuff to masturbate to. It's disgusting. In the end, he committed suicide by drowning himself in the reservoir.
The thing is, Kitae's father loved his son so much and couldn't accept his death, so he somehow started to build a cult and secretly made Kitae the "god" they worshipped. This cult was the same one Heesoo's parents were part of.
Becoming a worshipped being made ghost Kitae more powerful, and he was strong enough to lead Taeyung to his death by dragging him to the same reservoir.
Anyway, Taeyung actually loved Heesoo so much and somehow tried to protect Heesoo despite being a weak ghost. After his death, Heesoo was severely depressed and even thought about killing himself. One time he did attempted suicide, but ghost Taeyung saw it, so Taeyung tried to enter his friend's dream (the dude with a wife) and asked him to save Heesoo. The friend did that and Heesoo was saved, but didnt tell him about the dream he had.
Heesoo actually sent many messages to Taeyung before, telling him that he missed him so much and how he felt like he couldn't live without him. Maybe this was what made ghost Taeyung gain strength, because Heesoo wanted him to be there.
Anyway, the whole novel was very interesting and fun to read. The format is very unusual—it's in the form of chats and forum conversations (and sometimes twitter) from many perspectives, so I'm very amazed that the webtoon artist managed to adapt the scenes so well.
Jaekyung's personality is truly shit. I hope he'll lose the game and learn some lesson.