They Wasted A Perfectly Good Character / Literature
The English writer and literature historian George Saintsbury wrote of Éponine Thénardier in Les MisÚrables, claiming if "Hugo had chosen to take more trouble with her, might have been a great, and is actually the most interesting, character." Some adaptations (such as the Film of the Musical) cast an actress good enough for Éponine to become an Ensemble Darkhorse.
Carlisle is also a favorite among people who find the books otherwise horrific, mainly due to the way he actually makes good use of his condition. He's not used so much in the plot.
Jacob (generally before Eclipse and/or Breaking Dawn).
Leah is an Iron Woobie. She's trapped in a telepathic connection with her ex-boyfriend (who she shared a fulfilling relationship with before he Imprinted on Emily, Leah's cousin, and promptly ditched her) and a group of various people who all hate her because of her bitterness about the above. She's one of the few female characters who actively seeks freedom and independence, and she can transform into a wolf. Once Jacob's rebellion against the Quileute pack gives Leah an opportunity to escape the pack's mental link, she runs with it, leaving her family and protecting the vampires she detests, solely because doing so is more tolerable than being in Sam's proximity. (This is also around the time that she Takes A Level In Kindness and bitches out Bella for her cruelty to Jacob.)
Seth is a badass warrior, one of the series' few characters to be truly compassionate, and the only werewolf who doesn't hate vampires simply for existing. His main purpose in the narrative is as a sounding board for more angsty, pessimistic characters, and we don't get an idea of his perspective outside of that.
Rosalie Hale, Rich Bitch vampire with a distaste for Bella that many readers can sympathize with. (She's also hinted to be a Tsundere behind the scenes, and is one of the few vampires to see value in human lives). Her backstory is also incredibly interesting but it's glossed over in one brief moment in the third book.
Jasper is seen as a favorite amongst many: a battle-hardened vampire who was alive during The American Civil War and is now trying to become a better man due to falling in love with Alice.
Bella's father Charlie is also fairly popular among the people who otherwise generally don't like the series. Mostly it's the fact that he's one of few characters who seems to have real-world responsibilities and friendships, and the fact that, especially in the early novels, he openly distrusts Edward.
Many of the characters introduced in the last half of Breaking Dawn can be considered this, since they are only briefly introduced and never really explored. Some of their backstories, given in The Official Illustrated Guide are very interesting and could have made for good characters, had they been explored; not to mention improved the WorldBuilding by adding their experiences to those of the protagonists.
Leila from Fifty Shades of Grey. She is an ex-sub of Grey's who has been institutionalized twice against her will, attempted suicide and is portrayed as mentally unstable and out to get revenge on Christian or intent on harming Ana. She is the first one to point out to Ana that the two of them look very similar, the first sign that Grey specifically looks for women who have the same type of appearance that reminds him of his biological mother. Leila is treated as a big threat to the both of them, but sadly suffers from severe offstage villainy, which lessens her impact. When she actually does finally appear, threatening Ana with a gun, she is easily disarmed and carted off to another institute. Her entire arc took up 11 chapters with her actively only appearing in two of them, when she could have been expanded on into being a proper villain or even come across as a warning to Ana about what has happened to Christian's ex-subs and what could happen to her.
In The Riftwar Cycle the author devotes a book to novelizing the plot of the insanely successful RPG Betrayal at Krondor written by Neal Hallford and set in his world. One of the characters, Owyn Beleforte, ends up as a very powerful 19-year-old magician who has become friends with one of the allegedly Always Chaotic Evil dark elves and the first character to actually sympathize with the enemy. Additionally, he can understand their language due to a spell cast on him by one of their witches. He's also one of the handful in the world to be aware of the nature and location of the Artifact of Doom after helping save the world from it, as well as the super secret details of the last major war between humans and dark elves - to wit, the dark elves were manipulated by a third party into a near-suicidal invasion that killed many of them off like flies. Oh, and his dark elf friend dies through a Heroic Sacrifice. The story that practically begs to be told is that of Owyn taking up his fallen friend's cause and working further toward the peace which neither of the nations really want at the moment - he is unique in having both the backstory and motivation as well as the power to make feasible progress in it. The author, however, holds no interest at all in a character he didn't come up with, so after that book Owyn allegedly gave up the life of adventure and went home to live a normal life - according to Word of God - his fate didn't even get an in-story explanation, much less an appearance on-screen.
Gone has around 300 characters, and about 60 are named, so it's natural that there are a few examples of this, including Sinderm, a Perky Goth teenager with a green thumb and a blog, and Sanjit's siblings.
Henry, a vampire suffering from Creative Sterility, is given a name, a fascinating backstory and an interesting psychic power... and no speaking lines or any presence in the book.
Witkacy, Dora's partner in the police, a misplaced shaman who experiments with drugs, fights depression and sees ghosts - who come to him with their trouble - is unceremoniously shoved to the sidelines. At least he's getting his own book series.
Fans will never agree on it, but it seems that the decision to kill Anakin Solo just as they'd launched three major plot arcs around him (romance, check; special abilities, check; unique connection with enemy culture, check) was a bit of a dead end. Not to mention multiple Force prophecies about Anakin's future importance (some dating to well before the New Jedi Order era) that never played out. Oddly, the writers of the post-NJO era seem to agree, as they keep making everyone relive his death. Oddly, Anakin bordered on Creator's Pet in the Corellian Trilogy. According to the writers, they were going to make Anakin the hero of the NJO books, but George Lucas vetoed it because he was afraid people would mix up Anakin Skywalker and Anakin Solo.
Jaina Solo as well, since the writers turned her into a Flat Character.
Also, Anakin's girlfriend, Tahiri Veila, gets this; while she was important for much of the New Jedi Order, she got Demoted to Extra in the last book and then hovered around in the background for a while, as if the writers were unsure what to do with her - and then brought her back into the spotlight only to have a lot of her Character Development undone so she could be derailed into a villain. However, she later gets brought back for her own story arc in the Legacy of the Force.
Brian Jacques is guilty of this quite often in the Redwall series, introducing a charismatic, kickass new character and then killing them off within two chapters. Has overlapped with Too Cool to Live.
Among the characters that haven't died (yet), we have Robb Stark's wife Jeyne Westerling. The story of how they met is very fascinating on both her and Robb's part: she slept with Robb after he got wounded in a siege when he heard that Winterfell was sacked by his foster brother Theon Greyjoy. Robb, having inherited the Starks' tendency towards Honor Before Reason, broke off his engagement to his current allies, the power-hungry Freys, in order to marry her, and from what we know, the couple loved each other very much. Despite this, we get zero insight into her character beyond this and no details about her relationship with Robb.
Did anyone else plod through Brisingr just hoping that Eragon and Roran will go away so we can have more Nasuada chapters? It's amazing how awesome and realistic she becomes, probably because Paolini makes her problems practical concerns rather than the philosophical, "deep" issues he tries to have the others grapple with.
There were a few of these types in Animorphs, most notably Joe Bob Fenestre from The Warning and Arbat-Elivat-Estoni from The Arrival. David might count as well, given his much-anticipated return to the series was much less climatic than the fans were hoping for.
Remus Lupin, Nymphadora Tonks, Luna Lovegood, Mad-Eye Moody, Peter Pettigrew, the Hogwarts professors besides Dumbledore & Snape (not to mention the trope namer for Hufflepuff House, and Ravenclaw doesn't get much more exposure)... if we all thought up a character whose development we might consider to be short-changed and compiled them in one place, we'd probably have enough guests to hire out an entire restaurant for.
Despite being a pretty major character, Professor McGonagall has also been seen as this after Rowling revealed her terribly depressing backstory on Pottermore.
Theodore Nott, despite only being mentioned a few times a lot was revealed by Rowling about him. Apparently he's the son of one of Voldemort's early Death Eaters and is just as pure-blooded as Malfoy. He is very similar to Malfoy only Nott is not part of a Slytherin gang and may be even cleverer than him. He could've served as a foil to Malfoy but unfortunately he was almost completely cut.
With the exception of Spink, all of Nevare's academy classmates completely disappear from the story after the first book of The Soldier Son. Especially Gord and his troubled relation to Trist seemed to have loads of potential, but they are never seen again.
Mary Watson, née Morstan, in Sherlock Holmes. Despite playing a major role in The Sign of Four, having quite a bit of genuine detective skill herself, and ending up married to Dr. Watson, she's never used again except in cameos, and during the Time Skip between "The Final Problem" and "The Empty House", she's killed off.
Cinna. One of the first things established about him is that he volunteered to be the stylist for the District 12 tributes despite it being a traditionally unwanted job, but we never find out why. Or why he doesn't have a Capitol accent, or why his fashion sense is so understated as compared to the rest of the Capitol, or... anything about him really.
Johanna once says that the mimickery of the Jabberjays can't hurt her because everyone she loves is dead - we never find out why. She also barely appears in the third book.
In Mockingjay Haymich mentions that her family was murdered on President Snow's orders because she refused to cooperate with his plans to make her into a prostitute
Rue also doesn't get much characterisation - just enough to make her an appealing victim. Thresh got it even worse.
Nor does Prim. As the sister of the main character, and a huge motivation of Katniss' actions in the first book, you'd think some insight into her personality and their relationship would be shown.
Lavinia, the red-headed avox girl who Katniss and Gale saw escaping from the capitol. We're never told why they were running, how they got to district 12, or anything of the like, and she's promptly killed off with almost no part to play at all.
The girl from District 4 was the only member of the career pack in the first book to not have any form of personality revealed and is killed early on. In the film she wasn't even a member of the pack and was killed early on.
"Foxface". Some of the other tributes from the 74th Hunger Games that remained nameless throughout the first book had their names revealed in the second, but despite being built up as a potentially interesting character in the first book, her real name is never revealed and the victors' trip to District 5 is completely glossed over in Catching Fire.
In Perry Moore's Hero, the protagonist Thom gets picked up at a gay bar by a slightly older young man and has his first kiss with him. Later, it's revealed that Thom's would-be beau is the supervillain Ssnake, who stands accused of murdering a beloved superhero at the time this was happening. When Thom reveals this to the public, it sets in motion a series of events that lead to the real villain's downfall and saves the planet from destruction. But sadly, we never see Ssnake again, even though he could have been a much more interesting love interest than Goran.
Clawface is one of the earliest in the series. He was formerly one of Graystripe's idols, but turned out to be an unrepentant villain. He also murdered Firestar's love interest Spottedleaf in cold blood, and he was the only character in the series capable of making Firestar completely lose it and try to kill him (which happened whenever Firestar so much as saw him). As well, prequel novel Yellowfang's Secret revealed that he was Yellowfang's sister's mate, and brother to Nightstar, the leader of ShadowClan after Brokenstar. Despite this, his personal connections to the characters are never explored, and after he's killed in Fire and Ice he's never mentioned again. Even when the entire fourth arc revolved around the past villains making a comeback after their deaths and he was confirmed to be among them, he never actually appeared in the series.
Brokenstar is possibly an even bigger wasted character than Clawface. He's supposed to be the villain of the first book, and has one of the darkest plots in the series: use Child Soldiers to make his Clan larger and stronger than the others, then sweep through them and destroy them so that he can be in charge. Also, he managed to drive out an entire Clan, a feat which no one has ever been able to replicate, and he has another Clan completely subservient to him. Despite this, he's never written as a real threat, and gets overshadowed by the much more generic Tigerstar. Even his eventual comeback didn't help him.
Jingo and her crew. A Clan-like group facing their own struggles in a city and harbouring a grudge against Sol? Would be cool if they'd so much as been mentioned since their introduction in Sunrise.
Gaunt's Ghosts, true to its Anyone Can Die nature, is filled with people with interesting personalities or histories, or could otherwise have had a huge impact on the story, killed off unceremoniously without warning. Perhaps the most galling though is Colonel Wilder, Reasonable Authority Figure who managed to keep the Ghosts together and become their new commanding officer after many of the main characters are presumed dead, who is killed off in the very book he's introduced in.
Stevie Rae, a compassionate Wide-Eyed Idealistwho is killed early into the story, resurrected through dark magic, and becomes the disillusioned leader of the red fledgings. The glimpses we get of her descent of insanity (and eventual climb out of it) are quite interesting.
Speaking of Stevie Rae, a whole novel could be written about the red fledgings (outcast from everything they've ever known, seemingly abandoned by God, yet staying sane and retaining their personalities...).
The father in Gone Girl. He's set up as menacing and confused in turns, with a definite hate-on for women...until he's killed off by old age in the last act. He would have been the perfect killer, but Gillian Flynn went in another direction.
Jane Gallagher from The Catcher in the Rye. She's built up by Holden to be a unique, intelligent, and genuinely non-phony girl, she might have had sex with his friend/rival Stradlater, and then...she never appears.
Prince Garrid from the Tales of the Frog Princess series. He's a Friendly Neighborhood Vampire that is apparently the oldest in his family of vampires and clearly has a lot of authority in the family, alludes to a vampire/werewolf war that apparently killed his parents, and despite his abilities and initial creepiness, is a loyal friend and ally to Emma. He's even married to her best friend! You'd think he'd get something to do for the plot, but... nope!