The potions master; he loathes Harry in particular and Gryffindor students in general. He's also the head of Slytherin House. Snape is especially cruel to Neville, his least favorite student, and spends much of the year threatening to poison Harry under the pretense of testing Harry's skill at brewing antidotes. Harry notices that Snape appears afraid of Moody and witnesses several instances throughout the year of Snape speaking with Karkaroff, sometimes on a first-name basis, about something growing clearer on his left arm. During Harry's trip into Dumbledore's Pensieve, he discovers that Snape was once a Death Eater and he later learns that Voldemort's Dark Mark is on Snape's left arm. However, Snape supposedly became a spy for Dumbledore, and Dumbledore makes it clear to Harry that he trusts Snape entirely. Harry struggles to believe this as Snape tries to accuse Harry on several occasions of putting his name in the Goblet of Fire and of breaking rules. Despite all of this, Snape does seem to be on Dumbledore's side when Voldemort returns and even shows Fudge his Dark Mark as proof that Voldemort has returned. When he learns that Sirius is allied with Dumbledore, Snape is livid but shakes hands with Sirius as Dumbledore requests.