Kevin Thorn was the single most powerful entity of the Fables universe. In his pen the whole of the universe is inscribed and whatever he writes with it becomes reality. His power is not limited to just creating or destroying, he can make it so everything never even existed in the first place, which is his ultimate goal in The Great Fable Crossover. His only natural foil is his brother, in his vicinity he loses his ability to concentrate and become frustrated with his own work.
He is exclusively committed to creating the best story he can muster, and would go to great extents to protect them, even allow himself to be diminished if that would save them. However, he wouldn't abide for the changes made to his stories and would rather start from scratch than rewrite them again.