Lord Westfall is the father of Chaol and Terrin Westfall, and the lord of Anielle, who swore loyalty to Adarlan and House Havilliard.
The lord of Anielle is indirectly mentioned in Throne of Glass, and is directly mentioned in Heir of Fire, and Tower of Dawn. He makes a physical appearance in Crown Of Midnight that shows his eldest son, Chaol, making a deal with him that if he stands by him in proposing to the King of Adarlan that Calena go to Wendlyn then he'll come back to Anielle and rule. This ultimately never happens.
He had spent most of his life defending the villagers from the cruel, mountain tribes of White Fang Mountains. This has changed after the conquest of the local freefolk, bringing peace to the people of Anielle, and lord Westfall could finally focus on developing their welfare.
His son, Chaol, had left the title of his heir, in order to become the captain of Rifthold's palace guard and stay near to his best friend, prince Dorian. In the process, Chaol's younger brother, Terrin had become an heir to the throne.