I have chosen Caliban to discuss, since, as an actor, I find him the most interesting character and thus the most enjoyable to discuss.
Caliban's function in the plot is one that is difficult to define. He is not the key protagonist, since this title belongs to the treacherous Alonso in his usurpation of Prosporo. Infact he does not at all directly encourage the conclusion of the play.
Caliban has many small but essential functions; one of which is to create Shakespearean comic relief in his drunken trio with Trinculo and Stephano. He also creates contrasts with other characters, such as Caliban's association with the "earth" and evil magic (by being "got by the devil himself upon thy wicked dam" who is Sycorax, a which). This is contrasted with Ariel whose very name associates him with the air, and being a spirit he is also seen as a positive embodiment of the super-natural.
Caliban's lust for Miranda in "seeking to violate the honour" of her, is contrasted with Ferdinand's true love.
Miranda: Do you love me?
Ferdinand: ...I...do love, prize, honour you.
There are many suggestions in The Tempest' that give us clues into the character of Caliban such as being referred to continuously as a tortoise, fish, cat, monster and a misshapen knave, his very name has similarities to Cannibalism.
His mother being a witch does him no favours, but her treatment of Ariel (who we believe to be a "fine apparition" with his beautifully energetic language) certainly reflects badly on Caliban as a blood link, since she imprisoned Ariel in a "cloven pine...(for)...a dozen years". Then there is Caliban's attempt to "violate the honour of" Miranda; and at present not to be filled with guilt at this event but to say "would't had been done!...I had peopled else this island with Calibans". This certainly portrays Caliban as cold, evil and relentless that he would have repeated the rape.
Then when worshipping the drunken fool Stephano as a "God" and promising to show him "every fertile inch of the island", which is infact the same mistake he made with Prosporo, as he explains in Act one: "I showed thee all the qualities o' th' isle....Cursed be I that did so!". And now he makes the same mistake.
Then promising Stephano that Miranda "shall be thy bed" and asking him to "brain" Prosporo "and with a log batter his skull" it is clear there is certainly an evil side to Caliban.
He is also guilty of a credible proportion of the deadly sins:
Prosporo:There's...business for thee....
Caliban:I must eat my dinner.
He is thus guilty of gluttony and sloth, and in his attempt to "violate" Miranda he is guilty of lust. He also rather blasphemously worships Stephano's "liquor" as the bible.
However, their is a distinct otherside to Caliban in his soliloquy
"Be not afeared; the isle is full of noises,
sounds and sweet airs, that give delight, and hurt not."
It is certainly clear here that their is a more sensitive Caliban, not only in his concern for others in reassuring them that there is nothing to fear, but also in Shakespeare's alliteration of the s' sound to create a soft and sensual mood, and the use of words that portray a more emotional Caliban such as full, sweet, airs, give and delight. This emotional side also brings to light that Calibans hate for Miranda and Prosporo (ie, what portrays him as generally evil) is only to the fault of a building hatred by continual slavery and imprisonment. This is supported by a clear suggestion that before he was abused he also could appreciate emotion: "When thou cam'st first, thou strok'st me, and made much of...