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Cho Chang is introduced during the Third book as the Seeker for Ravenclaw's Quidditch team. Harry Potter describes her as being pretty, though he makes no move to pursue her during the novel. Upon first seeing Cho, Harry realizes that she makes him stomach gurgle, but is yet to realize a crush is forming.
In the fourth book, Harry has a crush on Cho. He wants to ask Cho to the Yule Ball, but once he finally gets his courage up to ask her, she is already going with Cedric Diggory. Cho dances with Cedric a lot at the ball, and Harry watches them the whole time.
In Harry Potter and the Order of the Phoenix, Harry shows romantic feelings for Cho, and shares his first kiss with her after a Dumbledore's Army meeting before the Christmas break. As Cho was crying, Harry described the kiss to Ron and Hermione as "wet.". The trio begins to have a discussion about how Cho felt and if Harry was a good kisser or not.
Portrayals within films
Cho Chang is portrayed primarily as Harry's love interest in the films, played by Katie Leung. She is introduced in the fourth movie, and causes much tension between Harry and Cedric.
In the fifth movie, she is a member of Dumbledore's Army and is the one who rats out on them to Professor Umbridge. Though, it later revealed that she was under the influence of Veritaserum, and not entirely at fault.
Cho is often described as being pretty, especially from Harry's perspective. She is a Seeker, which implies that she has a slight, athletic build and is quick on her feet (or broom). Her names indicates that she is of Chinese ancestry.
Cho is also told to have long, swishy black hair. She is portrayed to be averagely pretty form Harry's eyes.
Unfortunately, Cho is mostly personified through Harry's interest in her. She's a Ravenclaw, so she is supposedly intelligent. She also likes to play Quidditch. She's rather popular, as she's always seen with a group of Ravenclaw girls.
After Cedric's death, she becomes slightly erratic, often bursting into fits of tears. According to Hermione, she was so anxious that she was faced with the possibility of being kicked off of the Quidditch team. She seems unable to deal with Cedric's death, and attempts to have Harry as a replacement, unable to process the feelings that she may or may not have towards him. It's likely that she feels some obligation, as a popular and pretty girl, to have an equally popular boyfriend. In the end, it seems that Harry and Cho are able to have an awkward friendship, intensified through their efforts in Dumbledore's Army.
After breaking up with Harry, Cho still is in the belief that boys should be in her life; and in The Half Blood Prince, she is told to be "talking animatedly with Roger Davies...maybe too animatedly?"
Magical accomplishments and skills
Cho Chang graduated from Hogwarts in 1995. She was a member of Dumbledore's Army and fought against Voldemort during the Final Battle.
The only reference to Cho's parents is that they told Cho to stay on Umbridge's good side, but Cho didn't do as they wished, after what had happened to Cedric. Cho Chang ended up marrying a Muggle.