Who is Ariel?

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.