After Virgil and Dante enter through the gate of Hell, Dante is greatly perturbed by a hideous cacophony of screams, moans, and shrill, faint voices. He asks Virgil where these terrifying voices are coming from. Virgil says that these are the shades of those sinners who have "lost their intellect." This circle of Hell is occupied by those who failed to choose between good and evil, the "coward angels" who stubbornly refused to take sides in the eternal battle between God and Satan. One of these unfortunate characters is Pope Celestine V, whose abdication paved the way for Dante's personal and political enemy, Boniface VIII, to sit on the papal throne.
In Canto III, we're also introduced to the grotesque figure of Charon, who has the unenviable job of ferrying dead souls across the river Acheron. When he sees Dante and Virgil, he refuses to take them across. After all, Dante isn't yet dead. But Virgil soon persuades him otherwise, and Charon reluctantly takes them on board, allowing Dante and Virgil to continue on their epic journey.
The new characters are the Uncommitted Souls that Dante and Virgil see when they enter the Vestibule of Hell. These were people who were not committed to God on earth because they couldn't make a decision for good or evil. Their punishment is to be stung by wasps and hornets as they rush about, and worms feed on their blood that drips to the floor.
The next new character Dante meets is Charon, the one who ferries travelers across the Acheron River to Hell. He reminds Dante and Virgil that those who enter Hell do not return, but Virgel tells Dante that this doesn't apply to Dante since he's still alive.
In this canto, as in most cantos, there are several characters. The main characters are Dante and Virgil, his guide. Charon, the ferryman from Greek mythology who transports the dead, is in the canto, as are a large number of uncommitted souls. (They aren't really characterized individually.)