"Ooh, sticking up for Longbottom? ...Never thought you'd like fat little crybabies, Parvati."
-- Pansy Parkinson (PS9)
Described by Harry as “a hard-faced Slytherin girl” (PS9) and as having a face “like a pug” (PA6, GF27). She was the ringleader of a “gang of Slytherin girls” (OP25). Like Draco, Pansy tended to pick on and try to intimidate those who were weaker — or just different — than she, and often backed up his gang. In fifth year, Pansy and her gang proudly wore Draco’s “Potter Stinks” buttons to Potions class, and laughed at Hermione when she was accidentally struck with Draco’s Densaugeo spell that made her teeth grow to a hideous size (GF18). Hermione found some justice the following year when Pansy, who was a member of Umbridge’s Inquisitorial Squad, was hit by an Antler Jinx and had to miss class (OP30). The jinx may have been retaliation from someone in Dumbledore’s Army because Pansy was the student who entered the Room of Requirement and found the list of D.A. members for Umbridge, which led to Dumbledore being sacked (OP27).
Pansy showed considerable concern when Draco was gored by a hippogriff during their first Care of Magical Creatures lesson (PA6), and was Draco’s date for the Yule Ball (GF23). In their sixth year, Harry used his Cloak of Invisibility to spy on the Slytherin train car and saw Pansy stroking Draco’s hair. She seemed to be in awe that Draco was talking about becoming a Death Eater and leaving Hogwarts (HBP7). When Harry dueled with Draco and hit him with the Sectumsempra curse, Pansy visited him in the hospital then “lost no time in vilifying Harry far and wide” (HBP24).
Pansy’s true colors – not to mention her sheer stupidity – showed through the night of the Battle of Hogwarts, when she heard Voldemort’s demands for Harry to be turned over and yelled for someone to grab him. McGonagall made sure she was the first one out the door (DH31).
Just talents and skills typical of your basic mean-girl bully.
Other canon notes and references
J. K. Rowling has some strong opinions about Pansy Parkinson, the girl bully:
Let my girls be Hermiones, rather than Pansy Parkinsons. Let them never be Stupid Girls (JKR)
Q: Did he (Draco) graduate? And who did he marry? It wasn't Pansy right, or was it?
JKR: No! God, it wasn't Pansy Parkinson. I loathe Pansy Parkinson. I don't love Draco but I really dislike her. She's every girl who ever teased me at school. She's the Anti-Hermione. I loathe her. Yeah, sorry! Sidetracked there by my latent bitterness. He married Astoria Greengrass. (PC/JKR2)
Possible name origin
A pansy is a kind of violet. In magical terms, it is noteworthy for its appearance in Shakespeare's play "A Midsummer Night's Dream," in which it is referred to by its other name of "love-in-idleness." In the play, Puck used the juice of the pansy to cause most of the trouble, squeezing drops of it onto the eyes of unsuspecting sleepers to cause them to fall in love with the next creature they saw. We do not know if the juice of the pansy has similar properties in the Harry Potter universe, however. -- MLW
Played in the movies by Genevieve Gaunt (IMDb)
The Tragedy of Pansy Parkinson: Unlike other Slytherin characters such as Draco Malfoy or Regulus Black, Pansy Parkinson didn't change and grow much over the course of seven books, and she remained the girl version of Dudley Dursley - who at least changed enough to admit he would miss Harry by the end. Pansy, on the other hand, wanted to hand Harry over to Lord Voldemort. Perhaps it would be more precise to say she is a younger version of Bellatrix Lestrange.
And in her stupid eagerness to betray Harry and let Voldemort win the final battle, Pansy painted all of Slytherin House as cowards and "bad people" in a stereotyped way, even though obviously some of the students were first- and second-year children who barely knew any spells, and who couldn't fight anyway. We don't even know if any Slytherin students were left in the school who were over over 17, except for Draco, Crabbe and Goyle (who were hiding). McGonagall ordered the Slytherins to leave the Great Hall with Filch, and while most of the Ravenclaws left too, they were not considered cowards or traitors like the Slytherins.
So unfortunately, any reader hoping for a meaningful reconciliation of the Hogwarts Houses was disappointed that Pansy was viewed as the ultimate Slytherin spokesperson at the end, even more than Snape and his memories, or Regulus and Narcissa and their bravery. JKR was asked about this, and explained that the Slytherins were just trying to protect themselves - which must include Pansy, a character JKR has said she loathes. The author's explanation is less than satisfying since there was no actual mention of Slytherins rushing back to fight, only Slughorn and some unnamed reinforcements from Hogsmeade - no Slytherin students at all.
Question: And how much is it that being sorted into Slytherin is sorted into good guys and bad guys...
JKR: They're not all bad. I know I've said this before. ... well, far from it, as we know, at the end-- they may have a slightly more highly developed sense of preservation than other people, because-- A part of the final battle that made me smile was Slughorn galloping back with Slytherins. But they've gone off to get reinforcements first, you know what I'm saying? So yes, they came back, they came back to fight. But I'm sure many people would say, well that's common sense, isn't it? Isn't that smart, to get out, get more people and come back with them? It's the old saying, there is no truth, there are only points of view. (PC/JKR2)