From BookJive

Quirinus Quirrell is the Defence Against the Dark Arts teacher at Hogwarts during Harry's first year. Prior to his employment at Hogwarts, he is said by Hagrid to have had a "brilliant mind", and was a fine teacher while studying from books; some time before Harry's arrival at Hogwarts, "he took a year off to get some first-hand experience". Upon his return, he appears perpetually nervous and has developed a stutter and nervous tics. Quirrell's attire includes a new purple turban which he claims to have received as a reward from an African prince for getting rid of a zombie.

Harry first meets Quirrell at the Leaky Cauldron, while being escorted by Hagrid to Diagon Alley to shop for school supplies. Quirrell is next seen at Hogwarts conversing with Snape at the start-of-term banquet, and then regularly while teaching Defence Against the Dark Arts lessons. During the school's Halloween banquet, Quirrell appears in the Great Hall to warn staff and students of a troll in the dungeons, and then promptly faints. Harry, Ron, and Hermione come to suspect that Snape is on a mission from Voldemort to steal the Philosopher's Stone from a secret chamber in Hogwarts, where it is hidden. When Harry finally arrives in the chamber, he discovers that it is not Snape, but Quirrell who is the real villain. During the climax of the story, as Harry and Quirrell struggle to recover the stone from the Mirror of Erised, Voldemort reveals himself on the back of Quirrell's head, formerly concealed by the turban, and speaks directly to Harry, threatening to kill him if he does not assist Voldemort in recovering it. After Harry refuses, Voldemort orders Quirrell to attack Harry, who holds off Quirrell long enough for aid to arrive, at which point Voldemort flees, in his non-physical form. Voldemort's departure, as well as the agony suffered by him because of his contact with the morally pure Harry, causes Quirrell to die.

References

Wikipedia