I'm not sure as I haven't read it. But usually, the main characters would be the two best friends themselves.
Since we don't have more information about this specific 'best friends graphic novel', it's likely that the main characters are the best friends. They could be two girls or two boys or even a boy - girl duo. Their personalities would be distinct, like one being more adventurous and the other more cautious, which would create interesting dynamics in the story.