Since I haven't had the chance to read 'One of Us is the Killer Graphic Novel', it's hard to say exactly who the main characters are. However, it's likely that there are characters who are at the center of the mystery. There could be an ordinary person who becomes the key to solving the case, a mysterious stranger who seems suspicious from the start, and perhaps some friends or colleagues of the potential victims who are also involved in the story.
I don't know the specific main characters as I haven't read it. But typically, there would be a diverse group of characters that include suspects, possible victims, and maybe a detective - like figure trying to solve the mystery.
Without having read the 'One of Us is the Killer Graphic Novel', it's a bit of a guess. But usually in such stories, there will be a protagonist who is trying to figure out who the killer is among their group. There might be a character with a shady past who is an obvious suspect at first. And then there could be a supporting cast of characters like a nosy neighbor or a local authority figure who all play a part in either hindering or helping the search for the killer within the world of the graphic novel.