Since I haven't read 'The Fall of Rome A Novel', it's difficult to name the exact main characters. However, it's likely that there are characters representing different factions in Rome. There could be noble families trying to hold on to power, military leaders who are either defending or undermining the empire, and maybe some religious figures as well, given the role of religion in that era. Also, characters from the invading forces would play a significant part in the story, as their actions were crucial in the fall of Rome.
I'm not sure specifically as I haven't read the novel. But typically in a novel about the fall of Rome, you might expect characters like Roman emperors, generals, senators, and perhaps barbarians who were involved in the events of that time.