There are often the curious ones. These are people who have heard about the Slender Man and are drawn to find out more, which usually gets them into trouble. There are also the ones with a special ability, like being able to sense his presence before others. But no matter who the other characters are, the Slender Man is the most important. His actions and the fear he instills in others are what make the fanfiction what it is. He's like a shadowy puppet master pulling the strings in the story.
Bart would probably be both scared and curious. He might initially try to play pranks on Slender Man, but as things got scarier, he'd realize he was in over his head. Lisa, on the other hand, would likely try to research Slender Man to find a scientific or logical explanation for his existence.
Ruby would probably be both curious and cautious. Her adventurous spirit might make her want to learn more about this strange new entity, but her sense of responsibility for her team would keep her from being too reckless. Weiss would likely be initially dismissive, thinking it's some kind of trick or a new enemy that can be easily defeated with her skills and Dust magic. Blake would be more on guard, her past experiences making her more aware of hidden threats. Yang would probably be the first to want to throw a punch, seeing Slender Man as a direct threat to her and her sister.
Sometimes, the Slender Man might show up in horror-themed cartoons. But it's not that common and can vary depending on the specific cartoon series.
The dragon and the kyuubi are obviously the main characters. The dragon is often depicted as strong, with the ability to fly and breathe fire or other elements. The kyuubi, on the other hand, is known for its fox - like appearance and powerful magic, especially illusion - based magic.
Often, the main character is an exiled pokemon itself. It could be a Pikachu that was exiled for accidentally causing a power outage in a major pokemon city. Another common main character could be a trainer who is sympathetic to the exiled pokemon and decides to help them.
Often, the main character is the gamer themselves. They are usually a self - insert type of character who brings their gaming knowledge and skills into the Remnant world. They might be a bit of an outcast at first, as they are different from the regular inhabitants.
Often, you'll find dragon - riders as main characters. They are the ones who have a special bond with the dragons. There are also powerful mages who might interact with the dragons, either as friends or foes.
Sometimes, other gods and goddesses make an appearance. Athena might show up to collaborate with Hephaestus on a project. Their interactions can add a lot of depth to the story. For example, they might have different ideas on how to create a new weapon, and their debates and compromises can be really interesting to read about. And of course, there could be a villainous god who tries to steal Hephaestus' secrets related to his snaps.
Vegeta is often a main character in dark saiyan fanfiction. Given his prideful and sometimes volatile nature in the original series, it's easy to imagine him being swayed towards the dark side. Writers might explore what would happen if he fully embraced his darker impulses.
There are also cases where historical figures themselves are main characters in a different context. For example, a famous historical hero who is taken out of their own time and dropped into a future or an alternate past. They have to adapt to the new situation while also dealing with the fact that they are out of their own time.