Ariel is Prospero's "tricksy" spirit servant and attends to Prospero's every need. Unlike Caliban, Ariel has a (mostly) warm and loving relationship with Prospero, who saved Ariel when he arrived on the island. (The evil witch Sycorax imprisoned Ariel in a tree because the "delicate" spirit didn't have the heart to do her bidding.)
Even though Ariel is affectionate toward Prospero, we learn early on that Ariel isn't a servant by nature; he primarily wants his liberty, but, knowing that it will come, serves Prospero wholeheartedly and happily.
Ariel is notable for his use of white magic in the play, but also for his empathy and goodness. These traits are lacking in some of the play's human characters, and Ariel's feelings only make that fact more conspicuous. Most telling is his report on the three traitors: Antonio, Sebastian, and Alonso. He claims that their state is so pathetic, if Prospero saw them he would be moved to mercy and sympathy. Ariel thinks he himself would have that same tenderness, were he human. While we are reminded that this is a spirit of a not-human nature, he seems filled with angelic grace—even about human matters.
Check out Ariel's response when Prospero asks how the King and his party are doing:
Your charm so strongly works
That if you now beheld them, your affections
Would become tender.
Dost thou think so, spirit?
Mine would, sir, were I human.
And mine shall. (5.1.21-27)
Whoa! Did you notice what just happened? Prospero's just transformed from a revenge thirsty magician to a human being with the capacity to forgive his enemies and feel "tender[ness]" toward those who betrayed him and exiled him to the island. In other words, Ariel's compassionate spirit is the catalyst for Prospero's change. Without Ariel, Prospero may never have learned that "the rarer action is / In virtue than in vengeance" (5.1.35-36).
Ariel performs all of his services with great skill and presentation. From showing up as fire on the ship to his appearance as a great harpy to the three traitors, Ariel treasures the aesthetic. He tends to speak in beautifully poetic verse, even about the silliest things, without ever seeming foolish. Even as he pulls on Prospero's robes, he sings a beautiful little song. Ariel stands in for all that is delightful and good in the world.