Prospero is happy on his island. He has everything he wants here: his books, his daughter, and a couple of servants to do his bidding. But there is one pain in his heart, and it's not the obvious one: his daughter has grown into a beautiful woman but her life is incomplete. She must return to the world. And he cannot do that if he takes his revenge on his brother; the world does not look kindly on murder, even for revenge.
Ariel is the spirit of magic, more an aspect of Prospero's own power given personification than a separate life-form. Ariel is Prospero allowing himself to be free, and in the end Prospero must cast himself off to fit into society.
Miranda is the point about which the play turns. Prospero gives up his island, and his revenge, for her. She's grown up a little wild, without a mother to explain things to her. She's respectful and curious, but not really suited to the real world, which is where she belongs, not Prospero's private retirement island. She knows what she likes and takes it, and that's Ferdinand.
Ferdinand has grown up in the court of Naples and knows that he's going to be king. Surprisingly, this has not gone to his head, and he's turned into a sensible guy with a sense of honor and duty. He's turned down a half-dozen totally inappropriate marriage arrangements, and his father is getting pretty worried about him. He's a worthy target of Miranda's affections, and he sees in her the sort of woman he's been looking for: beautiful, self-confident, and "outside the system".
He knows that there's only one thing in the world worse than having power: not having power. His tenure as King of Naples has been a balancing act, trying to get the various bits of evils his dukes do to balance out. He's brought up his son with a strong sense of duty because he's the one who's got to try again.
The good man in a bad world. He's seen the botch that politics makes of lives and he wants to fix it, but he's learned that the bad guys move faster than the good ones. In his heart he believes that what goes around comes around.
Greedy, manipulative, self-absorbed, and hungry for power. He pressures Sebastian to kill Alonso to get power over him, as part of Alonso's way of seeking the kingdom of Naples for himself.
Not nearly as skilled as Antonio at playing the politics game, hampered by a conscience too big to let him move quickly and too small to really save his soul. He doesn't really want to be king of Naples, but having grown up in the court all his life he figures it's expected of him. He's actually jealous of Prospero's happy little life and would frankly rather remain here on the island.
Stephano is the guy in your high-school class who had a fake ID and brought the booze. He's the guy who would run for class president and get a big write-in from the stoner vote, if the stoners would bother to write. In my class, he actually won, and then got kicked out for not actually doing anything.
Trinculo is the guy who gets his beer from Stephano in the high school parking lot; the hanger-on. He's both jealous of Stephano and thinks he's the greatest thing since sliced bread.
Caliban is neither a monster nor a "noble savage". He's in fact, a very real savage: crude, rude, and socially unacceptable. He's the class clown. Think Jar-Jar Binks after a half-dozen whiskey sours. Do NOT do the voice. He inherited no magic power from his mother, who was a pretty crummy witch and a not-so-great mother, either.
The Spirits are shadowy figures. Nobody gets a really good look at them. They are not of this world, and they don't move, look, or act like people.
He's seen far too many storms to believe that this one is normal. He takes it as a challenge, and he's actually pretty happy to pit his skills against whatever it is nature is throwing at him. He's bold and makes decisions quickly and effectively.