Quirinus Quirrell is the hidden secondary antagonist of the book and the film adaptation of Harry Potter and the Philosopher's Stone.
He was the Defence Against the Dark Arts Teacher during Harry Potter's first year at Hogwarts. Throughout most of the film, it's believed that Severus Snape was the one who was after the Philosopher's Stone, however at the end it's shown that Quirrel was the true culprit and that he was sharing his body with Voldemort to allow the survival of the Dark Lord. His primary goal was to obtain the Philosopher's Stone that was hidden inside Hogwarts to give a new body to revive Lord Voldemort. He ended up being killed upon attempting to touch Harry Potter to get the stone because of the protection spell casted on Harry after the death of his mother, Lily Potter.