Get ready for some exposition from Doha's side in chapter 40.
To be honest, I didn't get the full picture either. It's a bit convoluted. Let me share what I understood anyway. So, Doha had previously inserted a new cassette tape into the black cassette player and played it, thus becoming a player character. Doha has now revealed that there was already another cassette tape (the one with the check mark on it) inside it. Normally, the cassette tape ejected from the cassette player dissappears in the next run. Doha noticed that the cassette tape with a check mark on it didn't disappear. So, he concluded that someone must have left this mark on purpose. He also deduced that that someone must be Bada's cousin, Seongchan, the reason for it being the massive change in his character since the first game Doha remembers. Normally, player Seongchan never entertained this horror thing but after turning into an NPC he talked about going on a ghost hunting in the abandoned house and whatnot. Player Seongchan always went outside to take a phone call and became the first victim of the killer. The reason he'd been doing this could be because Bada would be making progress in the game anyway without Seongchan having to do anything, or he could've been faking his death. The biggest giveaway, however, was the change in his behavior during the prologue, which Bada wasn't fully aware of since he was dozing off in the backseat. Player Seongchan pocketed the cassette tape ejected from the cassette player in the car to secretly use it to save the game later. But once he turned into an NPC after Doha used the black cassette player, he did not do it again. The cassette tape stayed in the car's cassette player, saving the game for Hyungshin Hyung (the good-natured idiot) who had inserted it into the car's cassette player. But, there appears to be something wrong with this save point because Hyungshin only remembered stuff from the most recent run not everything. Since Hyungshin's involvement messed with their gameplay, Bada and Doha decided to remove the cassette tape from the car, thus preventing Hyungshin to remember stuff from then on. This is basically what I understood from all this. If you have any more questions, I'm probably not the one with the answers haha.
He who falls into the sea embraces the snake