What is Virgil's relationship with Dante and his role in Dante's Inferno?

Virgil acts as Dante's guide on his journey through hell to find spiritual enlightenment. Virgil is a fitting guide for Dante as Dante admired Virgil's work and considered him to be the most sublime poet of all earlier poets. He aspired to write like Virgil and considered him to be the perfect example after which to model himself and his works. 

Virgil is able to accompany Dante through hell in which he warns him he will see many frightening, disturbing things. Virgil, however, is unable to guide Dante past purgatory becuase he was a pagan -- he lived before the time of Christ and worshiped the gods of mythology.  Because he didn't develop a personal relationship with Christ as savior, he is unable to dwell in heaven.

What role does Virgil play in Dante's Inferno?

Virgil's primary role is to serve as Dante's guide through the hell. Dante, at 35, is halfway through life and feels he has lost his way. With the help of Beatrice, Dante gains what no mortal man has: entry into Hades. Virgil's guiding him through the inferno at the request of Beatrice is meant to help Dante return to a faith-centered life. Yet Virgil warns him that the trip will not be easy:

I have come to lead you to the
other shore; into eternal darkness; into fire and into ice.

Virgil represents reason, and this makes him a wise and knowing guide who Dante very much looks up to as he explains the meaning of the various circles of hell. He also acts in many ways as a literary device, moving Dante and the action onward when they threaten to get bogged down in one place. In addition, he helps Dante, and therefore the reader, understand how to interpret hell, scolding him, for example, for being sorry for the damned for the punishments they are suffering.

Yet Virgil also has his limitations. He is the pinnacle of reason, but he is from the pagan world, so he does not fully understand matters of faith. He is a great sage when it comes to human reason, but he pales beside Beatrice when it comes to divine knowledge. Although he been blessed by God for the good life he lived on earth, and has a place in Limbo in the Hall of Kings, he is not a Christian, and therefore, cannot lead Dante into Paradise.