Well, the main character is often the reader themselves, as it's a reader insert fanfic. But there might also be other supporting characters. Maybe a mysterious stranger who seems to know more about the horror situation than they let on, or a friend of the reader who gets dragged into the spooky events.
Since it's a reader insert fanfic, you are the main character in a sense. Alongside you, there could be a group of friends or acquaintances that are also involved in the horror scenario. There may be an old hermit - like figure who has some knowledge about how to deal with the horror but is reluctant to share it until the situation becomes dire.
The main character is, of course, the inserted reader. Then there could be a villainous character that is the source of all the horror. This could be a demon, a deranged killer, or some other malevolent entity. There might also be some side characters like a local who warns the reader about the danger in the area but is ignored at first.