Quirrell

Professor Quirrell was the first Defence Against the Dark Arts teacher, but was fired after being possessed by Lord Voldemort and having his body destroyed.

Stone
Quirrell first appears at Diagon Alley, where he is seen amongst the crowd. He also appears at Hogwarts', where he is revealed to be the Defence Against the Dark Arts teacher. He seems very shy and frightened all the time. However, when the staff find a defeated Troll in the girl's lavatory, Snape suspects Quirrell, who has the ability to communicate with Trolls. When everyone leaves, Quirrell looks disappointed at the fact that the Troll is defeated.

At Harry's first Quidditch match, Quirrell jinxes Harry's broom in an attempt to kill him. However, Ron and Hermione think it's Snape jinxing the broom, and they set Snape's cloak on fire, which breaks Quirrell's eye contact as well, saving Harry.

Later, the trio see Quirrell go to the room with Fluffy the three-headed dog suspiciously and follow him. They realise that he wants the Philosopher's Stone (what Fluffy is guarding) and go through all the obstacles. However, Ron trips over and only Harry and Hermione face Quirrell. Quirrell is furious at the Mirror of Erised as he cannot get the Stone, then gets Harry to look in the Mirror. When Harry gets the Stone, Quirrell reveals he is possessed by Lord Voldemort and fights Harry. However, at Harry's touch, Quirrell's body begins to disintegrate into green vapour. First, Quirrell/Voldemort loses his arms, then as Harry touches his face, his whole body is destroyed, thus killing him.

Luckily, Quirrell is resurrected by school nurse, Madam Pomfrey, and put in a full-body cast.

Chamber
Quirrell is fully healed and is still a staff member. However, during all the petrifications, Quirrell resigns and leaves the school and goes to the Leaky Cauldron, stating that it is too dangerous at Hogwarts'.

Prisoner
When Security Trolls are installed at Hogwarts' to prevent Sirius Black from breaking in and murdering Harry, the trio go to Diagon Alley and asks Quirrell how to communicate with Trolls. Quirrell refuses, however, stating that even though it was the Dark Lord possessing him two years ago, he still despises Harry and a little bit of him still wants him dead.

Goblet
Quirrell is seen at Diagon Alley, but has no dialogue and is just a background character.