Who is Ariel in The Tempest and how did he become Prospero's servant?

Ariel is an airy sprite or spirit that Prospero finds on the deserted island. Ariel was too good-hearted to cooperate with the "blue-eyed hag," the sorceress Sycorax. In revenge, she trapped him inside a cloven pine for twelve years, then died. As the text explains:

Into a cloven pine, within which rift Imprisoned thou didst painfully remain A dozen years; within which space she died And left thee there, where thou didst vent thy groans ...

Who is Ariel in The Tempest?

Ariel is a character in The Tempest by William Shakespeare. He is not human but rather a magical being, a type of air spirit. His gender is actually somewhat ambiguous. Although in Shakespeare's period, all roles were played by men, subsequently Ariel has often been played by female actors.

Ariel had lived on the island before Prospero arrived and was imprisoned in a tree for twelve years by the evil witch Sycorax, the mother of Caliban (a bestial or monstrous character). Prospero freed Ariel from the prison and promised that after a period of servitude, he would give Ariel freedom, a promise fulfilled at the end of the play.

Ariel has many magical powers, including the power to command winds and storms which he uses to cause the shipwreck that brings Alonso and Antonio to the island.

"Ariel's Song" is considered among the most beautiful poetry of the play and has been set to music by several composers. It is also the origin of the expression "sea-change", a term first found in English in the lines:

Full fathom five thy father lies;
... Nothing of him that doth fade,
But doth suffer a sea-change
Into something rich and strange.

Ariel is a spirit of the air in Shakespeare's The Tempest. When the banished Prospero first arrives on his island exile, he discovers that Ariel has been imprisoned in a pine tree by the evil witch Sycorax. Prospero frees Ariel, for which the little spirit is immensely grateful, as one can well imagine. As an expression of gratitude, Ariel becomes Prospero's loyal and faithful servant. Like his new master, Ariel has magical powers, which he proceeds to use in accordance with Prospero's wishes. For instance, it is Ariel who whips up the storm—the tempest of the play's title—which causes the King of Naples and his crew to become shipwrecked on the island.

When Prospero frees Ariel from his imprisonment in the pine tree, he promises that he will one day give him his freedom. In the meantime, however, Ariel remains as Prospero's servant. Although Prospero treats Ariel a good deal better than Caliban—which isn't really saying all that much—he never lets him forget who's boss, even threatening at one point to imprison him for twelve years in an oak tree if he doesn't stop complaining:

If thou more murmur’st, I will rend an oak
And peg thee in his knotty entrails till
Thou hast howled away twelve winters (act 1, scene 2).

Despite, or because of, Prospero's bullying behavior, Ariel continues to serve his master diligently, but it's only when he impresses Prospero that he is finally released. This happens after Ariel uses his magic powers to make Ferdinand and Miranda fall in love. The forthcoming nuptials between his daughter and Ferdinand mean that Prospero can now return home to Milan. As he has no further need of Ariel's services, he makes good on his promise to give Ariel his freedom.