Since I haven't had the chance to read 'mooncop graphic novel', it's hard to determine the main characters precisely. However, given the title, the central character is likely to be the Mooncop. There could be supporting characters such as fellow officers, scientists working on the moon, or perhaps some alien - like beings if the story has a sci - fi slant that involves extraterrestrial life. These characters would interact with the Mooncop and play important roles in the development of the story.
Not having read the 'mooncop graphic novel', we can only make educated guesses about the main characters. Obviously, the Mooncop is a strong candidate for a main character. He or she is probably the one driving the story forward, facing various challenges on the moon. There might be a mentor - like figure for the Mooncop, someone with more experience on the moon who gives advice. And then there could be a group of characters who are in some sort of trouble that the Mooncop has to help. This could include miners on the moon who have stumbled upon something dangerous, or a group of lost astronauts. All these characters would contribute to the overall plot of the graphic novel.