In such an extensive series with a huge array of characters, movies only meant to be two and a half hours were bound to miss integral parts of the Harry Potter books. Storylines were cut to make room for Harry's most significant plotline. This choice hurt aspects of the films for their storyline, but it also didn't favor the characters themselves.
While some character adaptations are recognizable, others were severely wronged. Some characters were missing vital traits to fit the chosen storylines in the movies. Some could get away with the missing storylines, still having enough of their book counterparts' spirit to pass. Meanwhile, others weren't so lucky, and between reduced screentime and cutting out several parts of the books, the movie counterpart did not hold up to the novel's portrayal.
Similar: Severus Snape
Thought of as skulking around the castle like a bat, Alan Rickman's performance as Severus Snape is very reminiscent of the novel's counterpart. Snape is very recognizable in the two mediums as Snape shows just as much loathing for Harry and James Potter in the movies as he did in the novels. While the books show Snape's distaste of Sirius Black and Remus Lupin far more, Harry Potter and the Prisoner of Azkaban does a good job of showing Snape's dislike of the two other men. The movies do not hold back on Snape's personality, allowing him to be very similar to how he is written in the books.
Missed The Mark: Ron Weasley
Ron Weasley is Harry's best friend and a vital part of the central trio. However, the movies do not show all there is to his character. The films use Ron mostly as comic relief or by showing his worst traits. They do not show his best actions, letting other characters take the spotlight over him.
Ron is a funny person, but he is more than that. Ron has his own beliefs that get ignored in the films to portray Hermione as the main genius. When Ron does have his moments in the films, they don't come across as organically because of how they had written for him.
Similar: Hermione Granger
Even though Hermione Granger isn't identical to the book's version, she does have plenty of similarities. Like her book counterpart, Hermione is brilliant and excited by magic's capabilities. She is a good student and still has faults, even if they aren't as prominent in the movies as they are in the books. This works for Hermione because she needed to be so smart to understand several clues about the Deathly Hallows and Horcrux hunt.
Missed The Mark: Remus Lupin
This is a prime example of what can happen when so much of the novel's storyline is cut from the films. In the books, Remus has a bigger role in Harry's life and has a subplot story. In Harry Potter and the Deathly Hallows, Remus and Tonks reveal they are married and have a child on the way, and that Remus has been working undercover with the wolves. Except, there is no build-up to their relationship in the movies. Remus having fear over his child becoming a werewolf and wishing to abandon Tonks to join Harry on the Horcrux hunt is not a part of the movies, which is a major part of Remus's actions in the final installment.
Similar: Dolores Umbridge
Even though all of Umbridge's vile acts don't make it into the movies, film fans don't have to read the books to hate Dolores Umbridge. She is one of the truly despicable characters in the Harry Potter franchise. Even with her film counterpart not having identical actions to her book's portrayal, she is so awful that the movies do a solid job making her easily hated.
As nothing about Umbridge is redeemable, she is one of the most hated characters in the franchise, even more than Lord Voldemort.
Missed The Mark: Percy Weasley
Similar to Remus Lupin, Percy's character suffers from a lack of screentime. Percy is not close to Harry in the books, but he is far more part of the Weasley family drama. As Percy moved up in the Ministry, he became attached to Minister Fudge, siding against his family on several occasions. Percy's betrayal caused hurt feelings amongst the Weasley family until Percy's reunion during the Battle of Hogwarts. In the movies, Percy is shown beside Minister Fudge. Still, the conflicts with Percy's allegiance and how he hurt his family, along with his own personal transformation, are left out, leaving him not to be remembered well at all.
Similar: Lord Voldemort
One of the most intriguing things about Lord Voldemort is that fear over his name is so large that the Dark Lord doesn't need to appear much. Voldemort only appears at the end of the books and movies. Since there was less screentime to worry about, Voldemort was closer to his book counterpart. There was less time to fill for Voldemort, so he could be written to be more closely identified with his novel counterpart. The most massive difference between the two versions of Voldemort occurs during the Battle of Hogwarts. Voldemort's death scenes are vastly different, with the book Voldemort lying dead on the floor, mortal once again, while the film's version turns to dust.
Missed The Mark: Ginny Weasley
Like her brothers, Ginny doesn't match her book's portrayal. Ginny doesn't have the most massive role in the books either, but her characterization and actions show a much more fleshed out character. In the books, Ginny develops into a force to be reckoned with. Ginny uses magic on her brothers, is a part of Dumbledore's Army, and works with Neville against the Death Eaters at Hogwarts.
In the movies, Ginny appears as more of a background character. Some of who she is come through in the movies, but for the most part, it is a very different character.
Similar: Luna Lovegood
For a main character that doesn't appear as often as most others, Luna is still quite similar to her book counterpart. Luna is wise in a way where she can shock others with her intuition. She may make comments about Nargles, but she can also be relied on to stand loyally with her friends. In the novels, Luna helps Neville and Ginny cause chaos while Harry, Hermione, and Ron are away. Although that part of Luna isn't shown, she still has a perspective that helps shift Harry's mental state.
Missed The Mark: Harry Potter
Sometimes, even the main character isn't quite how they're meant to be. The films get Harry's angst, anger, and conflicts right. Harry faces several internal and external conflicts that bring out a bitter side of him. However, the movies spend so much time getting the angsty part of Harry's personality right that they ignore the rest of who he is. Harry is witty and sarcastic, unafraid to talk back to Vernon and Petunia after starting Hogwarts. Harry even gets snarky with Snape. These are comedic moments in the books that are left out. Writing them out of Harry's personality doesn't give a well-rounded portrayal of the book's titular character.