Azazel aka the Yellow-Eyed Demon (Fredric Lehne) was the thing that killed Sam and Dean's mother; he was the thing that stood over Sam's crib and dripped blood into his mouth, turning him into one of his future soldiers; he was the reason the Devil's Gate opened; and he was the reason, ultimately, that Dean sold his ...
The local sheriff helps Sam and Dean when he sees the Leviathan eating one of the officers and turning into Sam and Dean. Sam finds out Dean killed Amy, causing tension between the boys.