She guides Dante. She lectures Dante. She introduces him to everyone. She even speaks for him. Plus, she knows everything under the sun about theology, and she's not afraid to share it. There are even several times when Dante refers to her as a mother.
He's only in the story for three cantos, but (like Beatrice and Virgil), he guides Dante through an important part of the Divine Realms, the Empyrean itself. He takes Dante on a tour of the Celestial Rose, naming each figure individually and describing how the Rose is organized. He explains why children are here among the blessed. And, he's helpful in guiding Dante straight up to God. How? Via Mary. His eloquent prayer to her is a lesson in itself on persuasion. Part flattery, part request, part reasoning, and part working the audience, it convinces the Queen of Heaven herself to help Dante.