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.
Professor Snape Character Timeline in Harry Potter and the Goblet of Fire
The timeline below shows where the character Professor Snape appears in Harry Potter and the Goblet of Fire. The colored dots and icons indicate which themes are associated with that appearance.
...whole and after a minute, Ron pokes Harry. They step outside where, curiously, they hear Snape and Karkaroff using each other's first names and talking about something that's getting clearer. A... (full context)
...and checks the Marauder's Map. Harry notices that a dot labeled "Bartemius Crouch" is in Snape's office and wonders why Mr. Crouch is at Hogwarts now. Curious, Harry decides to investigate.... (full context)
...them. Pansy Parkinson throws her copy of the magazine Witch Weekly at Hermione and while Snape isn't paying attention, she finds an article titled "Harry Potter's Secret Heartache." In it, Rita... (full context)
...Hermione accepted the invitation than with wondering how Rita Skeeter obtained this information. Noticing this, Snape takes points from Gryffindor, confiscates Witch Weekly, and much to their horror, reads the article... (full context)
...obsessed with catching Dark wizards. Ron insists that this is why Mr. Crouch snuck into Snape's office, but Sirius says this doesn't make sense--Crouch has a ready-made cover to spy on... (full context)
...several names. Crouch says that they've already caught all but one, but then, Karkaroff names Snape. At this, Dumbledore stands and says that Snape was a Death Eater, but turned spy... (full context)
...and Sirius except for what he learned about Neville. Ron is incredulous that Dumbledore trusts Snape. Hermione says that this is what Rita Skeeter and Winky were talking about in reference... (full context)
...that Fudge shouldn't have brought "it" inside as they burst into the wing, accompanied by Snape. Dumbledore appears and McGonagall shrieks that Fudge brought a dementor with him, and the dementor... (full context)
...the staff table. Madame Maxime sits with Hagrid and Karkaroff is absent. Harry wonders what Snape did for Dumbledore and wonders if Snape is really on his side. Dumbledore interrupts Harry's... (full context)