Severus Snape in Canon

"Hang on..." Harry muttered to Ron. "There's an empty chair at the staff table...Where's Snape?"

Professor Severus Snape was Harry's least favorite teacher. Harry also happened to be Snape's least favorite student. Cruel, sarcastic, and disliked by everybody except the students from his own house (Slytherin), Snape taught Potions.

"Maybe he's ill!" said Ron hopefully.

"Maybe he's left," said Harry, "because he missed out on the Defense Against Dark Arts job again!"

"Or he might have been sacked!" said Ron enthusiastically. "I mean, everyone hates him__"

"Or maybe," said a very cold voice right behind them, "He"s waiting to hear why you two didn"t arrive on the school train."

Harry spun around. There, his black robes rippling in a cold breeze, stood Severus Snape. He was a thin man with sallow skin, a hooked nose, and greasy, shoulder-length black hair, and at this moment, he was smiling in a way that told Harry he and Ron were in very deep trouble.

Chamber of Secrets
Chapter 5
The Whomping Willow