We know these things about him:
• He's half-human, having been sired on his witch mother Sycorax by the devil-god Setebos.
• He's ugly, and somehow deformed, based on the way Prospero and Miranda talk about him.
• He's capable of doing menial labor, but he doesn't like it.
• Stephano and Trinculo, when they find him hiding under a cloak, think he might be a dead fish or something. He has an "ancient and fishlike smell."
As I was thinking about designing him, I sort of vacillated between two dramatic demands the play has for Caliban. When he is cursing Prospero in soliloquy, I think Caliban has to seem genuinely malicious and a little frightening. But when he's been seduced by Stephano, Trinculo, and their bottle of liquor, he suddenly slides downhill from being the villain of the piece to being ridiculed by the ridiculous. (In this way, his character trajectory almost resembles Malvolio's. Think how fun it would be to see the same actor playing those two parts in repertory.)
You can see Caliban costumed all sorts of ways for different productions of The Tempest. In fact, he might be one of the most fun Google Image searches I've ever done. Some people really play up the half-devil heritage. Some play up the fishiness. There's an awesome-looking full-size puppet Caliban that seems to favor humor over menace. I've even seen someone trying to design a Caliban with Sendak's Wild Things as a base.
I wanted to discard any worries about actually putting a human being into the "costume," and just design the "real Caliban" that the stage actor would be trying to approximate. If I'd seen the really fun Arthur Rackham illustration before I started working on mine, I'm sure I'd have been influenced by it.
These doodles seemed a little too far in the direction of malice for me. I'd already done another version that I liked better:
There's not must malice in that version of Caliban, but I like the way he looks anyway. I especially like the distortions of his anatomy. It's fun to picture those big shoulders toting a bundle of logs.
But then I started thinking about this "sympathetic Caliban" in the drinky scenes, and in Browning's "Caliban Upon Setebos," pondering the nature of his deity and the nature of the misfortunes inflicted upon him by Prospero's arrival.