Harry’s first Defence Against the Dark Arts teacher is a clever young wizard who took a ‘Grand Tour’ around the world before taking up his teaching post at Hogwarts. When Harry first meets Quirrell, he has adopted a turban for everyday wear. His nerves, expressed most obviously in his stammer, are so pronounced that it is rumoured the turban is stuffed full of garlic, to ward off vampires.
I saw Quirrell as a gifted but delicate boy, who would probably have been teased for his timidity and nerves during his school life. Feeling inadequate and wishing to prove himself, he developed an (initially theoretical) interest in the Dark Arts. Like many people who feel themselves to be insignificant, even laughable, Quirrell had a latent desire to make the world sit up and notice him.
Quirrell set out deliberately to find whatever remained of the Dark wizard, partly out of curiosity, partly out of that unacknowledged desire for importance. At the very least, Quirrell fantasised that he could be the man who tracked Voldemort down, but at best, might learn skills from Voldemort that would ensure he was never laughed at again.
Though Hagrid was correct in saying that Quirrell had a ‘brilliant mind,’ the Hogwarts teacher was both naive and arrogant in thinking that he would be able to control an encounter with Voldemort, even in the Dark wizard's weakened state. When Voldemort realised that the young man had a position at Hogwarts, he took immediate possession of Quirrell, who was incapable of resisting.
While Quirrell did not lose his soul, he became completely subjugated by Voldemort, who caused a frightful mutation of Quirrell's body: now Voldemort looked out of the back of Quirrell's head and directed his movements, even forcing him to attempt murder. Quirrell tried to put up feeble resistance on occasion, but Voldemort was far too strong for him.
Quirrell is, in effect, turned into a temporary Horcrux by Voldemort. He is greatly depleted by the physical strain of fighting the far stronger, evil soul inside him. Quirrell’s body manifests burns and blisters during his fight with Harry due to the protective power Harry's mother left in his skin when she died for him. When the body Voldemort and Quirrell are sharing is horribly burned by contact with Harry, the former flees just in time to save himself, leaving the damaged and enfeebled Quirrell to collapse and die.
J.K. Rowling's thoughts
Quirinus was a Roman God about whom there is not much information, although he is commonly associated with war - a clue that Quirrell is not quite as meek as he appears. ‘Quirrell,’ which is so nearly ‘squirrel’ - small, cute and harmless - also suggested ‘quiver,’ a nod to the character's innate nervousness.
Once at Hogwarts, the stammering professor wears a large purple turban that gives off an odd odour, his classroom smells strongly of garlic and he shies away from offering details about the Dark creatures he claims to have defeated.
After the Gryffindor versus HufflepuffQuidditch match, Harry overhears Professor Snape arguing with Professor Quirrell. Snape appears to be pressuring Professor Quirrell to reveal how to get past the three-headed dog. Harry, Ron and Hermione spend the next few weeks trying to quietly encourage the professor.
When Harry enters the final chamber beneath the school he finds Professor Quirrell standing in front of the Mirror of Erised, trying to obtain the Philosopher’s Stone. His nervous manner is gone. As Harry questions him, he reveals the truth: that he has been trying to steal the Philosopher’s Stone all year, he is the one who tried to jinx Harry off his broom at the Gryffindor versus Slytherin match, and he was the one drinking unicorn blood for Voldemort in the forest. He unravels his turban to reveal that Voldemort has taken possession of him: the former Dark Lord’s head appears out of the back of Professor Quirrell’s. On Voldemort’s orders, Professor Quirrell uses Harry to get the Stone out of the mirror, and then tries to take the Stone from him by force.
Professor Quirrell’s attempt to take the Stone from Harry leads to his death: though he attempts to kill Harry on Voldemort’s orders, he is unable to touch Harry’s skin without suffering serious pain. Harry defends himself from Professor Quirrell by trying to grab any part of him he can reach, and falls unconscious just as Dumbledore arrives in the chamber. Voldemort flees, leaving Professor Quirrell to die.