Professor
Severus Snape
Potions master, head of Slytherin, and a teacher whose grudge against Harry’s father is visible from the first lesson.
Snape enters as a classroom antagonist: greasy hair, a talent for humiliation, and a preference for his own house. He knows potions the way other people know gossip. Harry hates him quickly, and the books let that hatred feel earned before they complicate it.
What is fair to say early: he protects the school in ways that do not look like kindness, and he is not a simple villain even in year one. The full accounting is a late reveal. This profile does not spend it.
Later in the story
His loyalties, his patronus, and the reason for the old grudge are answered in the last books. Treat fan certainty before that point as a reread privilege, not as a fact you owe a first reader.