When you're a franchise as massive as Harry Potter, you're going to have characters who appear in nearly every film. The main core characters were in each installment while important players like Dumbledore, Sirius Black, Voldemort, and Bellatrix Lestrange were all included in most of the films.
That doesn't leave much room for characters to stand out if they were only in a single movie. However, a handful of characters managed to make an impact in their lone on-screen appearance. Some were evil, some were hilarious, and others played a pivotal part in the overall story despite not popping up in multiple movies.
Rufus Scrimgeour
There was a lot of potential for Rufus Scrimgeour to be a significant part of Deathly Hallows Part 1. As the new Minister of Magic, he had the difficult task of taking over the role after Cornelius Fudge fumbled his position so poorly in the wake of Voldemort's return.
Getting a respected actor like Bill Nighy to play him only added to how good Scrimgeour could be on screen. Alas, he wasn't really around for much, though the scene where he gives Harry, Ron, and Hermione the objects left to them by Dumbledore was one of the film's highlights.
Madam Hooch
Most of the teachers at Hogwarts, other than Defense Against the Dark Arts professors, were around for the majority of the franchise. However, one who only popped up in the Philosopher's Stone and never returned was Madam Hooch, who taught the kids how to fly on their brooms.
Madam Hooch was played by Zoë Wanamaker, a veteran actor who actually refused to return to the sequels after saying the producers were underpaying actors. Considering her main role was to teach the kids how to fly, she wouldn't have done much besides officiating Quidditch in later movies. She made an impact in her solo appearance though.
Mrs. Figg
This is a character who actually was mentioned in earlier books than her on-screen appearance. Harry often hated spending his time with Mrs. Figg as she babysat him while the Dursleys would go off to do fun things that he was never invited to.
Her only film was Order of the Phoenix when she surprised Harry by popping up after he fought off Dementors. She actually knew about the magical world and it turns out that she was a squib who even knew Dumbledore. She was the source of some funny moments, especially when she acted as a witness at Harry's trial.
Viktor Krum
The world of this franchise truly started expanding in The Goblet of Fire as other schools came to Hogwarts to join in on the Triwizard Tournament. From Durmstrang Institute came Viktor Krum, who was an obvious choice to compete given his status as a Quidditch star.
Krum played a bigger role than just participating in the tournament as he struck up a romantic relationship with Hermione. His role managed to further the romantic possibility of Ron and Hermione, making him important. On top of that, he has a terrifying part to play in the final task while under the Imperius Curse.
Professor Quirrell
The Defense Against the Dark Arts position at Hogwarts was a revolving door throughout the series, with seven different teachers across the seven books. The first introduced was Quirinus Quirrell, a mild-mannered man who often stuttered and seemed like a coward.
Nobody suspected him to be the person Harry would face off against at the end of the Philosopher's Stone, making the reveal all the better. The idea that Voldemort had been living in his head was pretty frightening and only added to him being a threat.
Xenophilius Lovegood
As soon as she debuted in The Order of the Phoenix, Luna Lovegood was painted as the series' oddest character. It made sense that her father turned out to be just as abnormal as she was, though that is actually the trait that makes the Lovegood family such a treat in the franchise.
Xenophilius Lovegood is in The Deathly Hallows Part 1, first meeting Harry at the wedding at the start of the movie. His great scene came later when Harry, Ron, and Hermione went to him while searching for Horcruxes. He is devastated by the danger Luna is in and it's clear when he talks to the trio. Plus, he's around for the incredible animated look at the Tale of the Three Brothers.
Barty Crouch Jr.
There's no doubt that the history of Barty Crouch Jr. is much better explained in the books. That being said, he still manages to nearly steal the show in The Goblet of Fire and most people had no idea that it was happening the entire time.
Barty Crouch Jr. spent most of the movie in disguise as Mad Eye Moody, who was the Defense Against the Dark Arts professor at Hogwarts. The reveal of his identity came off well as he was a crucial part of Voldemort managing to return to his true form, making him one of the franchise's most successful villains.
Aberforth Dumbledore
In a way, Aberforth Dumbledore actually appeared in more than one movie. He was seen in a photograph during The Order of the Phoenix as a member of the original Order of the Phoenix. However, he actually showed up in The Deathly Hallows Part 2.
Aberforth showed up at the perfect time, saving Harry, Ron, and Hermione when they were in serious trouble at Hogsmeade. Although he protested that fighting was useless at that point, he helped the heroes and had contact with Neville and others at Hogwarts, showing he had some fight left in him after all.
Gilderoy Lockhart
The second Defense Against the Dark Arts teacher in the franchise was Gilderoy Lockhart. Given that The Chamber of Secrets is the longest movie in the series, Lockhart was given plenty of screen time to truly develop in ways that other one-off characters didn't.
His arrogant and smug nature was evident though his ineptitude was always clear for those who paid enough attention. It let the viewers know that something was off about this guy. He wasn't pure evil but he was a phony who didn't deserve his fame. Kenneth Branagh delivered a masterful performance as Lockhart that is among the funniest in the franchise.
Cedric Diggory
Long before he was Batman and even before he was Edward Cullen, Robert Pattinson portrayed Cedric Diggory in The Goblet of Fire. It's surprising that the character only debuted in the fourth film since he was apparently a hotshot student at Hogwarts.
Cedric was an interesting character because he was basically the golden child of Hufflepuff. A house that was so often overlooked had someone who was legitimately great and could win the Triwizard Tournament. He also proved to be a good person who was willing to help Harry at every turn. His death was arguably the most tragic in the series and established the darker tone going forward.