I'm not sure exactly, but they are likely to be the dragon masters themselves.
The main characters in the 'dragon masters graphic novel' would be the dragon masters, of course. They are the ones who will be at the forefront of the action. However, we can also expect other characters such as those who help or oppose the dragon masters. Maybe there are mentors who taught the dragon masters how to handle their dragons, or villains who want to steal the dragons for their own evil purposes. And the dragons themselves can almost be considered main characters too as they are partners to the dragon masters and have their own personalities and wills.
The main characters are probably the dragon masters. There may also be some important dragons that play significant roles, like a wise old dragon or a powerful young dragon that has some special abilities which are crucial to the story.