There are many who play vital roles in the chronology of the plot. Here are few specific examples:
Aeolus is the god of the winds and he appears only once. Because of a debt he owes to Juno, he agrees to force the fleet of Aeneas off course, leading them to North Africa, Carthage, and Dido. Without Aeolus, the fleet would have landed comfortably in Italy.
Ghosts are mostly peripheral characters in this epic, but are important to moving the plot forward. The ghost of Hector is the one to encourage Aeneas to leave in the first place. The death of Creusa provides insight into Aeneas' spirit and sense of loyalty. Virgil uses her ghost to show these things, but to also make sure that Aeneas still gets out of the city in time. And it is the souls of ghosts in the underworld, the ghosts that will again have life on Earth, that encourages Aeneas to keep going when all seems lost.
Finally, consider Amata a very significant character. While the interference of the gods is vital to encouraging the break between the native Italians and the Trojans, it is Amata's original disapproval of her daughter's engagment to Aeneas that opens the can of worms.