Warning: MASSIVE spoilers ahead. If you haven't read the last Harry Potter book, but plan to do so, turn back now
Throughout the entire Harry Potter series, there was one character who was the comic 'bad guy' - designed to be disliked. This of course is Snape.
We think that he is a Death Eater, working for Voldemort, with a personal disliking of Harry.
However, in the final book in chapter 33 'The Prince's (Snape's) Tale', that all changes.
Our entire view of Snape changes, and we learn he isn't who we thought he was. He wasn't working against Dumbledore and for Voldemort, but actually the other way round:
"Hide them all, then," he croaked. "Keep her—them—safe. Please."
"And what will you give me in return, Severus?"
"In—in return?" Snape gaped at Dumbledore, and Harry expected him to protest, but after a long moment he said, "Anything."
"Snape raised his eyebrows and his tone was sardonic as he asked,
"Are you intending to let him kill you?"
[Dumbledore]: "Certainly not. You must kill me."
"Severus Snape wasn’t yours," said Harry. "Snape was Dumbledore’s, Dumbledore’s from the moment you started hunting down my mother."
We learn all this when Harry sees the memories he gave him just before he died.
This is possibly the biggest twist in the entire Harry Potter series. It shocked many, and seems completely unexpected.
But maybe it shouldn't have been so unexpected...
Was there any evidence prior to the revelation that Snape is actually a 'double-agent' and not who we thought he was?