Professor Quirrell is the Defense Against The Dark Arts teacher at Hogwarts School of Witchcraft and Wizardry during Harry Potter's first year at the school. He's paranoid, timid, and frightened all the time. Or at least that's what he appears to be when the reader first meets him. There is no way he could be anything of substance.
Until, he turned out to be the villain. This is what made the first Harry Potter book so wildly entertaining. It challenges the very image of an antagonist, the villain. Thus, readers everywhere averted their attention to the character who was the most villainous, Professor Severus Snape. And all the while, Quirrell slipped in without our ever knowing it. That was the genius of JK Rowling's writing.
The twist. Professor Quirrell was shocking, manipulative, arrogant, and confident. The total opposite of his other self that was introduced in the beginning. What makes Professor Quirrell memorable is his contradictory nature.