Ariel’s role in the beginning is very significant, because he is the one who conjures the tempest that causes the shipwreck.  He does this at Prospero’s bidding, because he was once imprisoned in a tree on the island by the evil witch Sycorax, and Prospero freed him.  Ariel has been promised freedom from captivity a year early if he performs certain services for Prospero: creating the tempest which causes the shipwreck, setting fire to the mast, and making sure that all the ship’s passengers arrive on the island safely, but in separate groups.  Ariel, who is invisible to all but Prospero, then uses his magic to set up the encounters and events that occur for the rest of the play.