The Other in The Avengers (2012). Although he is established at the beginning as the servant of a greater villain (revealed at the end to be Thanos), he is quite shadowy, sinister, and enigmatic in that he is original to the film series. He merely serves as communication between Loki and Thanos, and messenger to the latter. He's later killed by Ronan in Guardians of the Galaxy, meaning he's not going to have his time to shine.
Lady Sif, brave, beautiful, badass goddess of war from Asgard played with verve and gusto by Jaimie Alexander, is woefully underused in both Thor films, despite being an important character in the comics. At least she gets the honor of being one of the few movie characters to make an appearance in Agents of S.H.I.E.L.D., and most fans/reviewers agreed her appearance practically made the episode. She's not even present in Thor: Ragnarok, although Word of God states that she had been exiled by Loki (disguised as Odin) for fear that she would figure out his secret.
Similar to Sif are the Warriors Three, Fandral, Volstagg, and Hogun. Despite being shown to be Thor's True Companions, the three were underutilized for the entire trilogy. This reached its apex in Thor: Ragnarok when all three end up quickly getting killed off, and while Hogun at least gets a Defiant to the End moment, Fandral and Volstagg aren't even lucky to get that much, as both are unceremoniously killed in less than a minute. To add insult to injury, Thor doesn't even learn of their deaths for the entire film.
Hawkeye is considered one of the most underused Avenger and superhero. In contrast to his fellow S.H.I.E.L.D. agents and other Badass Normal heroes, he doesn't have enough moments to shine (he spent most of The Avengers as a brainwashed pawn of Loki) and has the least amount of screentime (he didn't appear in any Phase 2 movie aside from Age of Ultron). Granted, his archery skills wouldn't be much use against world-ending aliens and robots, but he was absent from more grounded installments like Captain America: The Winter Soldier and Agents of S.H.I.E.L.D. that could have put him on a more level playing field. Subverted in Avengers: Endgame when Hawkeye gets to play a major role in restoring those erased by The Snap, including his wife and kids, the loss of which sent him on a Roaring Rampage of Revenge during the 5-year Time Skip killing various criminal organizations around the world as the sword-wielding Ronin. Marvel will finally put him in the limelight in his own Disney+ show - granted, it's one that involves Passing the Torch to the other Hawkeye, but at least finally Clint got some attention.
In Thor: The Dark World, Malekith the Accursed (along with his fellow Dark Elves) is considered a rather forgettable Generic Doomsday Villain. Given how the Dark Elves were on the brink of extinction, Malekith could've been a sympathetic Anti-Villain who could be used to question the morality of Odin and the Asgardians. Alas, this wasn't explored at all and any depth or sympathetic aspects of Malekith were removed in the final cut of the film.
The movie infamously had The Reveal where the Mandarin we've been seeing is actually a washed-up British actor named Trevor Slattery, and the real Mandarin is Aldritch Killian. This ties in with Pragmatic Adaptation to a degree, as the decision to change the Mandarin so radically was motivated in part by Marvel not wanting to offend the Chinese box office with the Unfortunate Implications of an extreme Yellow Peril stereotype, but fans of the comics were still displeased. A compromise was reached All Hail the King, in which it was revealed that there was actually a real Mandarin the MCU who was much closer to the comic version. This Mandarin will finally be revealed in Shang Chi And The Legend Of The Ten Rings.
Also Maya Hansen. Originally, she was going to be the true mastermind behind the Mandarin, not Killian, which would've opened up a number of new storytelling avenues, as well as give the MCU its first main female villain a full four years before Hela in Thor: Ragnarok. Instead, the part was rewritten at the last moment due to concerns that a female villain wouldn't sell toys, leading to Maya being unceremoniously shot and killed well before the final act of the movie.
Quicksilver dies during the final battle. This provoked backlash from some fans and audiences, as this was only his first real appearance after the stinger of The Winter Soldier. It also meant that he couldn't take part in the film version of Civil War, despite the interesting directions that could've been taken with his character. What makes it worse is that he's a long-serving Avenger in the comics, having joined the team back in the 1960s not to mention that this is one of the very few media portrayals that does not have him involved with the X-Men in any way.
Baron Strucker is basically an extended cameo and is murdered offscreen by Ultron at the end of the first act. A shame, as he had the potential to be a recurring antagonist and the MCU is now unable to delve into his rivalry with Nick Fury. At least The Other had the dignity of an on-screen death, which served to make Ronan not seem like a wimp, as well as giving us the chance to hear Thanos speak. Interestingly enough, his children seem to be making more of an impact on the franchise than he ever did, with his son Werner von Strucker introduced as a recurring role in Agents of S.H.I.E.L.D. and a number of characters bearing his surname in The Gifted.
Sharon Carter in The Winter Soldier and Civil War. She has less than 8 minutes in both films combined, despite being one of Cap's main partners in the comics, romantic or otherwise. Basically, imagine a Superman movie where Lois Lane is a waitress in the Starbucks across from the Daily Planet. She was also originally planned to appear in Infinity War and Endgame but had all of her scenes cut out and was changed to having been killed off by the Snap. At least she will be in The Falcon and the Winter Soldier, which could ironically make Sharon spend more time with the next two Captain Americas than the one she's associated with in the comics...
Brock Rumlow/Crossbones in Civil War. After a fairly full appearance in The Winter Soldier where he was used as The Dragon, the film's last shot of him was laid out on a stretcher and badly burned, a very obvious Sequel Hook setting up his rise as Crossbones (one of Cap's most recurring villains in the comics). Speculation ran high as to what his role in Civil War would be, particularly once the stills of Rumlow in his very impressive Crossbones gear leaked. And then we got the film itself, which opted to use him as the throwaway Batman Cold Open villain these films seem inordinately fond of using as a narrative device, with Crossbones conveniently disposing of himself via a Taking You with Me attack. An ignoble send-off to one of the MCU's few recurring villains.
Ulysses Klaue as played by the delightfully hammy Andy Serkis was introduced in Age of Ultron as a "powerful new player in the Marvel Cinematic Universe" three years before the hero he's most associated with, Black Panther, got his own solo film. A Freeze-Frame Bonus established that Klaue had a long feud with the Wakandan Royal Family — his great-grandfather was killed in the 19th century by the then Black Panther, he tried to kill T'Chaka at least once, and he managed to steal some Vibranium from Wakanda and actually got to keep his life and his loot — although he was branded for his troubles. All of this makes the audience believe he'll be the main villain and Arch-Enemy to T'Challa. In truth, he's actually the Disc-One Final Boss and something of a Red Herring for the audience, as he's betrayed and murdered by Killmonger, the film's real villain, partway through the second act. While Killmonger did end up being praised as one of the MCU's most complex and interesting villains, many still feel that it might have been a good idea to leave Klaue alive for future movies, especially given that his comic counterpart went on to join the Masters of Evil and clash with a wide variety of Marvel heroes. That Klaue died before he could even get his trademark sonic powers from the comics just rubbed more salt in the wound.
The Abomination from The Incredible Hulk is almost certainly the first and oldest of these characters in the MCU, as he appeared in one of the very first films in the series, and is noteworthy for being one of the very few villains to survive his film. Technically, he's still alive in canon to this day, but aside from getting a mention in The Avengers Prelude: Fury's Big Week (where Tony Stark (of all people) keeps him from being assigned to the Avengers by General Ross), the character has remained a Sealed Evil in a Can.
On the subject of the Hulk, Dr. Samuel Sterns, aka the Hulk's Arch-Enemy The Leader in the comics, has also ended up becoming wasted potential. Sterns is last seen being infected by some of the leftover irradiated blood and having his head begin to grow as he starts to transform into The Leader in an obvious Sequel Hook, but with the Hulk's solo film rights tied up with Universal, the chances of him showing up again are slim to none. He's mentioned as being dealt with and detained by the Black Widow in supplemental material but has otherwise been completely forgotten.
Double-whammy version for Minn-Erva in Captain Marvel (2019). In the comics, Minn-Erva was a Kree Hot Scientist and Eugenicist whose pet project was using Mar-Vell's genes to improve the Kree, creating an interesting connection between Minn-Erva and both Mar-Vell and Carol. The movie abandons this backstory, with promotional material describing Minn-Erva as the previous star player of Star-Force who feels threatened by Carol's power and competence. Despite either of these backstories providing plenty of depth for the character, the movie itself uses Minn-Erva as little more than another goon for the Kree empire, ignoring any potential character development she could have gotten. The nail in the coffin is that she seemingly gets killed off in the finale, preventing her from getting any future character arc to live up to her potential.
The rest of Star-Force falls into this as well. We dont really get to see much of Carols relationship with the rest of her teammates during her time as a member of Star-Force. At-Lass was pretty hesitant in trying to fight Carol, and she doesn't kill them despite showing that she could, implying that she had a pretty close relationship with her teammates aside from Minn-Erva. Some fans were also disappointed that none of them aside from Carol have superpowers like in the comics, which could make them much more of a challenge for Carol to fight. Instead, they are simply a group of Badass Normal and are quickly trounced by Carol.
Believe it or not, Thanos himself falls victim to this in Avengers: Endgame. He's killed in the first twenty minutes with little fanfare, completely throwing any further Character Development out the window, when he could have been shown that his attempt to "balance" the universe had not been Worth It, and thus be forced into a Redemption arc like his comics counterpart. Instead, he's replaced with his past self who has not undergone the sacrifices or hardships from Infinity War, thus coming across as more of a Generic Doomsday Villain and making the final battle feel far less personal.
Cyclops, leader of the X-Men and one of the main characters of the comics: the guy was less than a cameo. The character had to deal with the death of his lover, but apparently that wasn't worth exploring, nor were his, you know, actual leadership abilities. Also, with Xavier's death, the idea of Cyke taking leadership of the X-Men as a whole and not just the field team was very doable (he's been the leader of the majority of the mutants for years in the comics), and would've made sense in the context of the story. But no, he was thrown out like yesterday's trash just so Fox could have more of the eternally overexposed Wolverine. This was partly due to James Marsden opting to do Superman Returns which clashed with the filming of the third film - so he had to be written out.
The film wastes not only Colossus but Psylocke and Multiple Man as well. Even Callisto could have gotten more characterization mileage than simply being another of Magneto's lackeys. Basically, there are two kinds of characters in the third film: on one hand, you have the characters who could all be the poster children for this trope. On the other hand, you have Wolverine.
Wade Wilson/Deadpool. After getting the chance to display a weary snarky persona, he's taken out of the film until the climax, where we see that Stryker has basically turned him into an Humanoid Abomination. With his mouth sewn shut, essentially robbing him of the sole trait he had in the movie. Luckily, he gets much betterreceived movies later, the latter of which quite blatantly mocks his characterization in the film as it ends with Deadpool using time travel to kill his unpopular past self.
Comic fans had been calling for Gambit to show up in the movies since the first X-Men film. When he finally does here he's little more than a built-up throwaway gag.
The Wolverine: The decision to have the Silver Samurai be the old Big Bad guy, when they had TWO characters who could have made for a really badass Powered Armor warrior for Wolverine to face and have a real adamantium fight with (those two being Harada and Shingen) annoyed some fans.
Darwin, who was shown to have useful powers and a fair amount of character potential, falls victim to Black Dude Dies First in a relatively minor conflict well before the final showdown. Despite the fact that his power is specifically that he can't die and there is literally no reason given for why he did. Not only that, his power would have been a much, better template for the sentinels in X-Men: Days of Future Past than Mystique.
Riptide and Azazel, two of Shaw's minions who join Magneto after he kills Shaw, are quite popular among the fans, some of whom even ship them. Even their actors were interested in their return in X-Men: Days of Future Past, Alex Gonzalez said in an interview that he'd "really like to be in the sequel" and explore more of Riptide's powers, and having signed on to play him in one or two more films, while Jason Flemyng said he was interested in exploring the origins of Azazel's son, Nightcrawler. Yet plans for the Hellfire Club's return were dropped when the writers chose the Days of Future Past storyline, Azazel gets killed in-between movies and Riptide, who is never mentioned, presumably suffers the same fate.
X-Men: Days of Future Past: The only mutants in the future who get any meaningful dialogue are Xavier and Magneto, with the rest essentially being glorified cameos, even established characters like Storm, Iceman, and Kitty Pryde. This is mostly a result of several scenes cut from the film in order to keep the runtime short. Those hoping for more of the First Class mutants in future installments had their hopes dashed when nearly all of them were killed offscreen. Unless said characters are revealed to be Not Quite Dead (which certainly could happen with Emma Frost as she was not among the autopsies of confirmed dead mutants) or come Back from the Dead, they aren't going to be used again - which is a bit strange when you consider that this movie resurrected every other character.
Unlike previous installments (in which she made token cameo appearances, sometimes as a Damsel in Distress), Jubilee was intended to play a bigger role in Apocalypse. Lana Condor was cast to play the character and was a big part of the Viral Marketing campaign (appearing in an in-universe ad promoting the Xavier School for Gifted Youngsters). However, her role in the final film is minimal at best — she's in the background of several shots, goes to a movie with Scott, Jean and Kurt, then gets knocked unconscious by General Stryker and doesn't appear again until the very end of the film, after the final battle. It doesn't help that Condor herself mentioned that she had scenes showcasing her powers that were cut from the final product, as well as a longer mall sequence which only appeared in a TV spot.
Ironically, the film's titular Big Badhimself is regarded by many fans this way, since like the MCU adaptation of Mandarin above he was a very loose adaptation of the Apocalypse character who actually had more in common with lesser-known X-villain the Shadow King than with the Big Blue fans knew from the comics. The fact that he is given a very definite death courtesy of Phoenix Jean, killing any possibility he might return also factors into why fans consider him a waste of a perfectly good character.
Dark Phoenix: Many of the supporting Mutants namely, Quicksilver, Storm, and Nightcrawler aren't given much to do from a character development perspective. Quicksilver gets the worst of it, considering that Days of Future Past and Apocalypse teased that he would eventually reveal himself to Magneto that he is Magneto's son, but the two never share any scenes.
Django Unchained does this with a lot of characters. It was originally meant to be two films a la Kill Bill - but Tarantino decided to edit down to one film instead. As a result, many scenes and characters were Demoted to Extra or cut entirely. There is one character played by Zoë Bell who gets one lingering close-up, implying she is about to enter the plot in a big way - and then is promptly killed off. Considering how heavily the film runs on The Smurfette Principle, one does wonder.
Julio in Elysium, whose relationship with Max can serve as a foil to him, but he is quickly replaced by Spider.
The ninth film, Jason Goes to Hell: The Final Friday, reveals that Jason Voorhees has a sister. But instead of using this to tell the audience more about Jason or the Voorhees family's backstory, her role in the film is to be killed by him and having her corpse to be conveniently present for the climax.
On the subject of the Voorhees family, there's also Elias Voorhees, the father of Jason and Diana mentioned above. He was originally slated to make his film debut in Friday the 13th Part VI: Jason Lives, only for his scenes to get cut, though they were left in the film novelization giving readers an intriguing peek at his character. A man with a quiet and sinister presence not unlike his son, it is heavily implied that Elias knows what Jason is, as he paid for burials for both Jason and Pamela rather than a cremation which would have prevented Jason's resurrection. Additionally, at the end he is shown watching the lake, implying he knows his son is still alive. This is heavily downplayed in later adaptations, which tend to portray Elias as the stereotypical abusive hillbilly husband instead to give Pamela a Freudian Excuse for her madness.
Joe Brody, who is regarded by critics and fans as the movie's best human character, dies in the first third of the film. His son, Ford, who replaces him as the human protagonist, is viewed to be less interesting by comparison.
Godzilla himself never gets a real appearance until the final act, with the MUTOs driving most of the plot and is only present to combat the creatures' existence.
Sally Hawkins' character Vivienne Graham also has shades of "They Wasted a Perfectly Good Actress," since she doesn't have much to do except help Serizawa with plot exposition.
Godzilla: King of the Monsters (2019): There was a significant amount of backlash at the way Vivienne Graham was killed off. Despite the director's intention to use it to establish the KaijuBig Bad's evil streak, fans condemned it as poorly-written, too out-of-the-blue, severing the MonsterVerse's link to the 2014 film, and as an unjust or too-unceremonious end for her.
Halo 4: Forward Unto Dawn builds the backstory for young Thomas Lasky, future Captain of the UNSC Infinity, who learns to shoulder responsibility in the face of doom thanks to the example of Master Chief. As for his supportive best friend Chyler Silva, she gets promoted to his girlfriend for forty minutes before getting abruptly Stuffed into the Fridge. Halo 4 then introduces Sarah Palmer, the chief officer of the Infinity's Spartan-IVs who is alsoThe Lancer to Lasky.
Any character that isn't Eggsy and Harry in Kingsman: The Secret Service and its sequel. Other characters are either removed from the story or killed to give more screentime to the two main characters. Even when Harry is killed off two thirds into the first movie, he was brought back alive in the sequel at the expense of killing off other Kingsman agents including Roxy/Agent Lancelot and Merlin in rather cheap ways, and severely downplay the role of Statesman agents once Harry is back into the picture.
Mikaela Banes. Upon close examination, as done by film critic Lindsay Ellis, the first film can be considered Mikaela's coming-of-age story instead of Sam's. Mikaela has the more interesting backstory and character arc that fit into the "No sacrifice, no victory" theme. Unlike Sam, Mikaela can handle herself in situations, possesses skills useful to the Autobots, and she ends up a different character at the end compared to the beginning. However, Sam is the character the audience is introduced to first and therefore the one people are expected to follow and get invested in.
Maggie Madsen. She's the computer analyst who discovered the Decepticons hacking into Air Force One and the first to suggest the hacker can't be human but possibly a living computer-based organism. After her theory is dismissed by her superiors, Maggie seeks outside help from her hacker friend Glen but the act gets them both arrested. Once she brings Glen into the picture, Maggie is mostly relegated to a comic side character and her analyst skills are never used again.
In the backstory the Greek Gods (especially Diana's patron goddess Athena) are all dead save for Ares, who by his own admission destroyed them. A common complaint among Wonder Woman fans is that while she is an excellent character herself, her stories generally don't have the same density of World Building and Rogues Gallery that Batman and Superman have, and that DC rarely does heavy lifting in integrating the richness of Classical Mythology to its superhero lore the way Marvel does with Norse Mythology, and that arbitrarily wiping out the Pantheon, for the sake of simplifying Diana's origin, potentially limited the scope of her stories going forward.
Many bemoaned how brief Etta Candy's role was in the film, as she is Wonder Woman's traditional sidekick and she provides excellent comic relief and a glimpse into the early feminist movement.