Who is Gwystyl from The Black Cauldron and what is their importance?

Gwystyl is the keeper of a way post for the Fair Folk, located near Annuvin. He seems like a weak, grumpy, and cowardly creature, who only reluctantly agrees to help the companions. However, this seems to be merely a front, as Gwystyl is actually a lot more intelligent and brave than he lets on. He seems to have been putting on an act in order to encourage Taran and the others down the path they took, leading to the destruction of the cauldron. When they leave on their quest, Gwystyl immediately seeks out Gwydion to let him know, which allows Gwydion to come to the rescue at the end of the story.