Sybill Patricia Trelawney is the professor of Divination. She is the great-great-granddaughter of the celebrated seer Cassandra Trelawney, and, in fact, has inherited some of her ancestor's talent. Trelawney is described as a slight woman resembling an insect, draped in a large spangeled shawl and many gaudy bangles and rings. She speaks in a misty voice and wears thick glasses, which cause her eyes to appear greatly magnified. Her odd classroom in the North Tower of Hogwarts is a cross between "someone's attic and an old-fashioned tea shop"; it can only be reached by climbing to the top of the stairs and then up a ladder through a trapdoor set in the ceiling. This dim, heavily scented, and "stiflingly" warm room often affects students' wakefulness.
Trelawney first appears in the third book of the series, when Harry, Ron, and Hermione start divination lessons. The friends generally believe Trelawney is a fraud. However, Gryffindor students Parvati Patil and Lavender Brown are very fond of and impressed by her. According to McGonagall, her credibility as a seer is undermined by her habit of erroneously predicting, each year, the death of one of her students. However, from time to time she does make predictions within the books, which come true. Her more profound predictions seem only to happen when she is in a trance and unaware of what she is saying, with no memory of it afterward.
Prior to the events of the Harry Potter books, Trelawney falls into a prophetic trance while in an interview with Dumbledore at the Hog's Head, making a prophecy about the birth of a wizard "with the power to vanquish" Voldemort. This prophecy was partly overheard by Snape, who relayed what he heard to Voldemort. This led Voldemort to attack the Potter family, believing that Harry was the child named. In Harry Potter and the Prisoner of Azkaban, Trelawney prophesies to Harry about the events of the book's climax. In Order of the Phoenix, Trelawney is first put on probation by Umbridge, and later fired. However, Trelawney continues to reside in Hogwarts castle due to an order by Dumbledore, who believes that she would be in danger outside of Hogwarts due to the prophecy she made during her interview. Dumbledore later reveals that the prophecy is why he keeps her employed as a Divination teacher. Trelawney returns to teaching in Half-Blood Prince, but has to share classes with Firenze, as he was cast out by his fellow centaurs when he agreed to replace Trelawney after she was fired. In Harry Potter and the Deathly Hallows, she is seen aiding in the Battle of Hogwarts by magically accelerating her crystal balls at Death Eaters. She uses one of these crystal balls to defend an injured Lavender Brown against the attack of werewolf Fenrir Greyback.
In the British editions of the books, her name is consistently spelled as "Sybill". In the American editions, from her first appearance in Prisoner of Azkaban through Order of the Phoenix, her name is spelled as "Sibyll". However, in the American edition of Half-Blood Prince, it is re-spelled as "Sybill", matching the UK edition.
Trelawney is portrayed by Emma Thompson in the Prisoner of Azkaban, Order of the Phoenix, and Deathly Hallows.
IGN listed Trelawney as their 22nd top Harry Potter character and called her "fascinating and fun".