Ariel was the first person Prospero encountered on the island in the tale. He freed the spirit after finding Ariel trapped in a cloven pine tree. Prospero promised to regain his freedom in exchange for a year of loyal devotion from Ariel. We don't know how long Ariel has been working for Prospero when the play first begins. Given that he had been on the island for 12 years, Ariel may have worked for Prospero for a significant deal longer than what their employment agreement stipulated. It is also plausible that Prospero saved Ariel from the tree a year before the events of the play. In either instance, Ariel's immense power is the cause of Prospero's reluctance to free her.