Granite is the alias of Sir Tiergan, a Telepath who works with the Black Swan. His real name is Sir Tiergan, and is a member of the leading council of the Black Swan, The Collective. As mentioned in the book Neverseen, he consumed a chalky powder called Indurite, which causes his body to crystallize and harden, making him look like a talking statue; this fact is most likely why he is called Granite. Because he is Sir Tiergan, he is also in custody of Wylie. When Wylie's mom died and his dad's mind broke, Tiergan felt that it was his duty to take care of Wylie.
Granite is an anagram, meaning that if you take the word 'Granite' and scramble it, it could spell "Tiergan."