Which Roman poet joins Dante and Virgil in Purgatorio?

The Roman poet who joins Dante and Virgil in Purgatorio is Statius.

Statius appears in Purgatorio XXI after an earthquake announces that his soul is ready to ascend. He reveals that he has spent centuries purging his sins on Mount Purgatory, and he joins Dante and Virgil for the remainder of the ascent. His presence creates a remarkable meeting between two poets from different eras.

Statius is best known for the Thebaid, an epic about the war between the sons of Oedipus, and for the unfinished Achilleid. Dante also presents him as a great admirer of Virgil’s Aeneid. In the poem’s imaginative history, reading Virgil’s poetry helped turn Statius toward moral and religious understanding.

Dante’s Statius is portrayed as a secret Christian, although there is no firm historical evidence that the ancient poet converted to Christianity. This is a literary invention that allows Statius to serve as a bridge between classical poetry, Virgilian reason, and the Christian revelation represented later by Beatrice.