Ariel and Caliban are the most striking pair of foils in The Tempest. Ariel is an airy, ethereal spirit. He sings beautiful songs and bends nature to his will. He also conjures the tempest at the beginning of the play. Before the play began, Prospero found Ariel imprisoned and decided to free him, but now—because Prospero helped him—Ariel is compelled to serve him.
By contrast, Caliban is an earthy, plodding figure. He grunts, swears, and complains that he has to serve Prospero, who tasks him with mundane chores like fetching firewood. Prospero enslaves him and treats him very harshly. In Act 1, Scene 2, Prospero accuses him of trying to rape his daughter Miranda:
Thou most lying slave,
Whom stripes may move, not kindness! I have used thee,
Filth as thou art, with human care, and lodged thee
In mine own cell, till thou didst seek to violate
The honour of my child.
Because Caliban threatened to violate his child, Prospero treats him very differently. While he seems fond of Ariel, he refers to Caliban as a "slave" and "earth" and "filth." Interestingly, Caliban wishes he had been able to impregnate Miranda and "people[...] This isle with Calibans." By contrast, Ariel remains invisible to Miranda for much of her time on the island.
