Thimbletack

"You don't see us, now you do. But only if we want you to."

Thimbletack is the main supporting protagonist from The Spiderwick Chronicles.

Biography
Thimbletack is a loyal friend to Arthur Spiderwick and the Grace children. He warns them about the perils of the Faerie world and helps them with their mission. However, if he gets angry or betrayed, Thimbletack, like any house brownie, can turn into a boggart and make messes. If he is appeased, he can calm down and shrink back into a brownie.

Trivia

 * Thimbletack is voiced by Martin Short in the film adaptation.