Most probably the two brothers are the main characters. However, there could also be other supporting characters like their parents, friends, or maybe an antagonist who creates conflict for the two brothers. These additional characters would help to flesh out the story, add more depth to the brothers' relationships, and create a more complex and interesting plot.
The main characters are presumably the two brothers. But depending on the story, there could be other important characters. For instance, if the brothers are part of a team or a community, the leader of that group could play a significant role. Or perhaps there's a mentor figure who guides the brothers in their journey. Without reading the graphic novel, it's hard to know exactly who all the main characters are, but the two brothers are definitely at the center of it.
Since I haven't read it, I can't say for sure. But as it's called 'two brothers graphic novel', the two brothers are most likely the main characters.