Prospero can claim both Ariel and Caliban as his servants, but his attitude toward and treatment of the two are very different. Both became his slaves because of the machinations of the witch Sycorax, who had preceded Prospero on the island and was dead before he arrived. Caliban was her son; Ariel was a spirit whom she trapped inside a tree. Prospero, upon his arrival on the island after being banished from Milan by his own brother, freed Ariel from the tree in exchange for servitude. He also raised Caliban and taught him to speak and to behave in a human manner. However, when Caliban attempted to rape Prospero's daughter Miranda, Prospero spurned him and enslaved him. Prospero has promised to free Ariel from his bondage after Ariel helps Prospero carry out his plot to regain his rightful place in society and marry off his daughter, and he does so. Prospero also pardons Caliban and then breaks his staff and rejects his life as a magician, returning to Milan to live out the remainder of his days.
Caliban and Ariel have completely different views of Prospero. Ariel considers Prospero to be his savior and treats him with respect, carrying out his orders in the knowledge, or at least hope, that Prospero will keep his promise. Caliban hates Prospero for keeping him from Miranda and his plan to populate the island with little Calibans, so he does nothing for Prospero willingly and bears the hatred and scorn of both Prospero and Miranda. It seems as if both Caliban and Ariel are a little afraid of Prospero's powers and do not dare to cross him.