Avenger Whitehearth is the creation of Samantha Flynn (Booksbooks3). You can read more about them here.
Avenger Whitehearth is a Warforged assassin and spy, previously a member of the Fifth Crown known as Crown Shade. He is an accomplished archer, bordering on legendary, and when The Mourning struck he waded back into the mists to try and save his comrades.
He largely failed, but it was after one of these expeditions back into the Mournland that he ran into Avenger North and was recruited.