Quirinus Quirrell
Age: Died while still relatively young, although his age was not confirmed.
House: Unknown.
Distinguishing Features: An exceptionally nervous manner. He had a twitching eye, a stammer, and he looked rather pale more often than not. From 1991 onwards he wore a ridiculous purple turban which he used to disguise the fact that he had Voldemort living on the back of his head.
Heritage: Unknown. Possibly pureblood due to Voldemort's acceptance of him, although beggars can't be choosers and so the Dark Lord probably wasn't feeling particularly picky at the time.
Family: Unknown.
First Mention: Chapter Five of Philosopher's Stone/Sorcerer's Stone.
General: Professor Quirrell held the Defence Against the Dark Arts teaching position at Hogwarts until the end of Harry's first year. Renowned for his brilliant mind, he had a great theoretical knowledge of his subject, but craved genuine hands-on experience. Therefore, at some point before the 1991/92 academic year, Quirrell took some time off to gain first-hand experience putting into practice his knowledge of fighting against the Dark Arts.
The popular belief is that he travelled to the Black Forest where he ran into more trouble than he could handle in the shape of some vampires and a hag. As a result, when he returned to school to resume his teaching, he was afraid of everything, including the students themselves. He had also developed a stutter, which contributed to his weak and helpless appearance.
The truth was that he had in fact ventured to Albania where he had encountered Lord Voldemort, who was no doubt inhabiting the body of a small rodent at the time. This was certainly more trouble than Quirinus could handle - he was persuaded over to the dark side and he became the servant of the Dark Lord.
At some point during August 1991, Quirrell began wearing a purple turban on his head. It emitted a strange smell and it was speculated (primarily by Fred and George Weasley) that it contained garlic to ward off vampires. It in fact concealed the face of Lord Voldemort. The Dark Lord had become tired of his servant's failure to steal the Philosopher's Stone from the vault in Gringotts where it was stored, and had decided to inhabit Quirrell's body in order to exert greater control over him. It is unlikely that Voldemort was aware that this decision would bring him into direct contact with his nemesis Harry Potter, but it turned out to be a very happy coincidence for him, at least to begin with. He had possessed Quirrell to get his hands on the Philosopher's Stone, but he had gained the chance to get his revenge on Potter as well.
Quirrell served his master as best he could throughout the school year, although by his own admission he often fell short of the high standards demanded of him.
At the first Quidditch match of the year (Gryffindor against Slytherin), Quirinus hexed Harry's broomstick in an effort to get him to fall. However, Severus Snape - suspicious of Quirrell's loyalties from the start - performed a counter-curse, which kept Harry seated. This gave Hermione Granger enough time to create a distraction when she knocked Quirrell over in her attempt to reach Snape and set his robes on fire. She believed that it was Snape who was jinxing Harry rather than saving him, but her inadvertent push on Quirrell on the way resulted in him losing eye contact and breaking the spell anyway.
At Halloween, Quirrell allowed a troll into Hogwarts, creating a diversion while he attempted to get past the three-headed-dog Fluffy, who was guarding the trapdoor that led to the chamber of the Philosopher's Stone. He failed, much to Voldemort's annoyance no doubt.
At the end of the school year, he finally made a successful move. After managing to send Dumbledore off on a wild goose chase to the Ministry of Magic, he finally got past Fluffy. This he achieved by playing music to him on a harp, which sent him to sleep - an approach he had learned by tricking the ever-gullible Hagrid into telling him what he needed to do.
The motley assortment of games and traps set by the teachers below the trapdoor hardly delayed him for a moment, and soon enough he was in the chamber, standing in front of the Mirror of Erised. He hardly even seemed surprised that Harry had managed to follow him; he seemed rather pleased in fact.
There, Quirrell revealed to Harry that his stutter was false and was only used in an attempt to seem more innocent. He also explained that he served Voldemort, justifying himself by claiming that there was no good and evil, only power and those too weak to seek it. The final revelation came when Quirrell removed his turban and revealed the Dark Lord himself nesting in the back of his head.
Even at this late stage, however, Quirrell was foiled. Whatever he did, he could not get past the trap Dumbledore had set using the Mirror - the Stone would present itself only to someone who wanted to take it but not to use it. Therefore it appeared in Harry's pocket rather than Quirrell's. Voldemort realized straight away and ordered his servant to take it by force, but Quirrell found himself unable to touch Harry without sustaining serious burns to himself. When it became clear he could not retrieve the Stone, Voldemort left his body and Quirrell died shortly afterwards.