Seth Clearwater is a member of Jacob Black's Pack and was introduced in New Moon as Leah's younger brother and member of the La Push werewolf pack. After Jacob Black and his own sister Leah Clearwater, he is the most developed werewolf in the Twilight Saga. He is the son of Harry and Sue Clearwater, and is also the second cousin of Emily Young.
Seth changed earlier in his life than the other werewolves, and unlike the rest, he is friendly towards vampires.
Seth is first mentioned in New Moon when Bella meets him at a dinner party at Billy's house, where Seth hangs out with her and Jacob. Seth apparently looks up to Jacob, and states that he "hung on Jacob's every word with idolizing eyes." Seth interrupts Jacob whenever it seems like he might have forgotten him. His father Harry dies of heart attack near the end of the book, and it is believed it was caused by the shock of Seth's and Leah's phasing. Seth also attends his funeral, which Edward mistakes for Bella's.
Seth's role becomes larger in Eclipse. In the book, Seth is present at the clifftop bonfire where Bella hears the Quileute legends for the first time and pays close attention to the tales, leaning forwards eagerly. He later goes with Jacob, Bella and Edward to serve as a connection between the werewolf pack and the small camp, and fights alongside Edward against Victoria and Riley, the leader of the newborn vampire army.
As a wolf, Seth remains tall and gangly, and grows sandy colored fur. In the film version of Eclipse, his build was based more on a coyote's than a wolf's, smaller and rangier than those of the older wolves.