Spirit is the second member of the Lupine Kinsfolk. He is Stein's closest friend and he considers himself to be second-in-command. He is not.
Spirit constantly worries about Stein's mental health and often questions whether or not Stein is fit to lead the pack. Stein often neglects Spirit emotionally and puts very little faith in him.
Spirit joined Stein to form the Lupine Kinsfolk after he was chased from his former pack for being unfaithful to his mate. He became a pupsitter because he greatly misses his first and only daughter.