Warning: Spoilers may be unmarked. If you have not watched the latest season, proceed with caution!
Initially, Meg was this. Despite being sent back to Hell at the end of season one, many fans felt like her role ended much too early. She had a role in one episode of season two, but then she disappeared. It seems as though the writers felt the same way as she was brought back in season five.
Missouri Moseley, the psychic from the first season. She's an old friend of John's, is funny, has her head on straight, and has great chemistry with the boys that makes her seem like a stern grandmother. Despite all that, she only appears in one episode, then comes Back for the Dead in one episode of Season 13, where she gets Killed Offscreen after the first act. In fairness to the writers, they had intended to give the character a larger role in Season 1, but the actress was unavailable at the time.
Cassie Robinson, also from the first season, had great potential to be a love interest for Dean, as she was one of the few women from his past that he was shown to truly love, and he was the one who promised to return to her despite her insistence that it wasn't going to work. She never appeared again. This was especially glaring when Lisa Braeden was reworked as the girl of Dean's dreams who he goes to see before planning to say yes to Michael, and (temporarily) settles down with after Sam's Heroic Sacrifice. This is not helped by the fact that (like Missouri) she is black and her replacement is white, given Supernatural's general issues with race and representation.
Meg's "brother" Tom. He seemed just as powerful as her, but was much more cold and focused. He could've been an interesting contrast to Azazel and Meg's personalities. He played a large role in one episode, and briefly appeared in the following one where he was killed by Dean.
Uriel is a Smug Snake and complex villain, who turned against Heaven because of love for his brother Lucifer. He and Castiel are both rebels, in different ways, and he could have continued to be a strong antagonist.
Eve was an interesting and powerful new villain who was hyped up for most of Season 6 as the Big Bad. She could disable angelic powers, so Cass being overpowered wouldn't be an issue. Her being the villain would make even non-mytharc Monster of the Week episodes exciting, as it provided a reason why the Winchesters were hunting small-time cases instead of focusing on the Big Bad (they could be tracking down Eve's children to interrogate and such) and Eve could use her children to spy on them and also create new monsters the writers could come up with whole-cloth. Unfortunately, she appeared in a measly three episodes, and was then killed before the season was even over. There's also the fact that her motivations were actually sympathetic (she just wanted to protect her children) and had no real animosity towards mankind in general (though she is still somewhat sadistic as she did force even her more benevolent children to kill). This is never really explored at all.
Season 8 had this problem in multiple instances, due to having too many characters lying around and therefore killing a lot of them off when they're no longer important or convenient to have around. Examples include:
Samandriel, one of the few angels who wasn't a total jerk, gets killed by Castiel at Naomi's command after providing a bit of plot-relevant information. The "waste" comes in when you realize he would have been a great fit for the fallen angels plot that arises at the very end of the season.
Meg, the only remaining recurring character from Season 1 except for the Ghostfacers, had interest in Castiel, wanted to stop Crowley, and was considering redemption. Makes her a perfect candidate for the "cure a demon" trial, right? Except she's killed ingloriously by Crowley defending the Winchesters and Castiel before the trial in question is even introduced. (This is sadly a case of Real Life Writes the Plot; actress Rachel Miner has developed health problems that make it prohibitively difficult for her to adhere to a shooting schedule, though more recently she has said she would return if asked.)
Naomi, after being hyped all season as a villain who heads Heaven's Secret Service and had history with Crowley, does a brief Heel–Face Turn and is promptly killed off as her role as Castiel's manipulator and the focus of the Heaven portions of the arc are both hijacked by Metatron.
Benny. Badass Friendly Neighborhood Vampire, Purgatory veteran, and ally to Dean at a time when he didn't have many. But he was the center of a Sam/Dean conflict, so he had to go.
Kevin. He's gone from an Asian and Nerdy stereotype to losing everything in the span of one year; his future, his mother, and his girlfriend, and becoming a tired Knight in Sour Armour at a very young age, even outsmarting Crowley. In his last appearance, after calling Dean out for ruining his life, a possessed Sam murders him, because according to him "It's for the best". Granted he does then come back as a ghost and gets some closure with brothers saving his mother, and thus ends with the two getting to spend just a bit more time together before he passes on.
Ava Wilson has a great introduction as a completely normal person drawn into the story against her will, and is mysteriously kidnapped by the Yellow-Eyed Demon at the end, leaving all kinds of possibilities open. Then she returns, having gone through an offscreen Face–Heel Turn, and is anticlimactically killed off.
Andy Gallagher was also a well-liked member of the special children with an entertaining personality and powers and an Evil Twin only to be killed off in an even faster and more anticlimactic fashion than Ava after coming back.
Agent Henrickson proves to be a quite worthy ally after learning the supernatural is real, and could have been a great recurring character as he tries to further come to terms with it while keeping up his job. Instead, Lilith Dropped a Bridge on Him. Especially notable because the Word of God about why he was killed off doesn't even make sense: after the writer's strike cut the season short, they didn't want Sam and Dean hunted by the FBI for another season... except Henrickson had already cleared that up by the time he was killed.
Ellen and Jo. A mother-daughter team of hunters felt fresh considering the heavy focus on male bonds in the show and how almost every hunter who appears is a middle-aged white guy. Ellen was also one of the few Good Parents in a show sorely lacking them, a friend of John's who knew his history against Azazel well enough that she could've been used to give Sam and Dean information on the demon, was the boys' first mother figure in the show (later replaced by Jody), and balanced "tough" and "kind" perfectly thanks to Samantha Ferris' performance. Jo could've worked either as a love interest or a little sister figure, but as a character in her own right she saw hunting in a more positive light than most characters, stood out in that she wanted to be a hunter when even most hunters don't and while contrasting Sam and Mary who were White Sheep because they didn't want to be hunters, and was poised to be the first active female hunter on the show with her mom seeming retired; unfortunately, the beefed up badass Jo in Season 5 who has become a hunter barely has any time to shine, and even her more-experienced mom gets forced into the Damsel in Distress role in both instances we actually see her hunt ("All Hell Breaks Loose", "Good God Y'all"), with them only reuniting and teaming up to hunt together offscreen after Season 2. The Harvelles also are considered like family by the Winchesters, but have fewer appearances than you'd think given their early prominence in season 2. At least Ellen was supposed to return in Season 3, but this never happened thanks to the writer's strike and instead they missed two entire seasons, returning simply to be killed off in a Senseless Sacrifice for a plan that never had a hope of working (and incidentally, getting rid of the only recurring heroic female characters at the time).
Lenore, a Vegetarian Vampire in a constant struggle against her own nature, and played by the geek-adored Amber Benson. She appears in a grand total of two episodes, five years apart, and the second is only for a single scene where she's killed off. Some of that lost potential was later realized with Benny, but his own appearance in this list shows that for many it wasn't enough.
Malachi, another rogue angel who starts an Enemy Civil War with Bartholomew that actually injects some life into the plotline. And then it's just as quickly abandoned when he's killed offscreen after only appearing in a single episode.
Anna from Seasons 4-5, a kind, pretty angel who Took a Level in Badass and helped Sam, Dean, and Cas (even got into bed with Dean), loved humanity and wanted more than anything to be human herself with a unique backstory that had a great deal of potential to explore psychologically (as an angel who had been reborn as a human with human family and friends, with her past as an angel bleeding into her life as a human and causing her trouble), and is rumored to have been meant to be the brothers' angel ally. Her romantic relationship with Dean showed him being softer and more vulnerable with her than he usually is with his love interests, she was a rare voice of compassion and forgiveness he rarely got in his life, and she had the power and knowledge to handle any supernatural creatures that typically target Winchester girlfriends. Her Big Sister Mentor relationship with Castiel especially was a great angelic counterpart to Sam's and Dean's that was interesting, gave both characters great lines, and would've given Cas at least one non-hostile angel sibling to go on the run with while Sam and Dean were doing their human thing on Earth. Because the writers only wanted one angel ally, when Castiel proved to be more popular Anna was taken to Heaven and tortured both physically and mentally until she was forced to believe that the only way to avert the apocalypse was to kill Sam and Dean before they were born. Unlike others who Sam and Dean have fought to bring back to their side they show absolutely no hesitation about trying to stop her or talk her out of it and even allow her to die painfully. Note that when Castiel became god in season 6/7 he did much worse and was forgiven very easily.
Dr. Robert, a Creepy GoodBack-Alley Doctor with an even creepier assistant, who was apparently good friends with John Winchester (enough to give Dean a big hug upon meeting him) and knew how to induce astral projection, probably among other things. It took us until Season 6 to even learn he existed, and has yet to follow up after his first appearance (in fact, he disappears from the story in the first ten minutes of his debut episode). Probably justified because he was played by special guest star Robert Englund.
Frank Devereaux from Season 7, a crazy Conspiracy Theorist whose services as a coordinator and informant made him fairly important after Bobby died. Not too long after his introduction, he is implied to have been killed (his place is ransacked and covered in blood), but considering his usual mannerisms it wouldn't be too hard to write this off as him trying to cover his tracks, since they Never Found the Body (and why would Leviathans leave bloodstains, anyway?). Nonetheless, it seems he is dead for good and not due to make another appearance.
Cuthbert Sinclair, a former Man of Letters and a "master of Spells" who did a Face–Heel Turn due his perceived belief they weren't using the information they collected to its full potential, and was ejected from the order for a number of offenses including misusing information and creating a curse so sadistically cruel and lethal, it led to two other members dying to trying to stop it. He found an immortality spell that lets him still be an active threat in the present day, and has acquired a variety of rare and strange mystical artifacts, and a "zoo" full of weird creatures and monsters. He's played off as little more than any other random villain of the week despite having the potential to be another Dick Roman.
Cole Trenton, who could have provided a lot of insight into the collateral damage the Winchesters and other Hunters leave in their wake. Instead he's just a pretty cardboard villain for the first two episodes of Season 10, and then his resolution is jarringly crammed into the last few minutes of a completely unrelated episode, leaving many fans wondering what the point was. Dean even says he still has no idea what kind of monster Cole's father was, like the writers just refused to put any effort into the character's story at all.
Toni Bevell was absolutely right when she said that Sam and Dean had caused a lot of damage. It's almost as if the writers realised she had a valid point and decided to make her as psychotic as possible to turn the fans against her.
Ramiel. A demon picked and turned by Lucifer himself? Brother to Azazel, the Yellow-Eyed Demon? Collector of old and mysterious artifacts? Ties to Crowley and his rise to being king? Check to all of these! He's killed off the same episode he's introduced. Because of course he is. Though it may be They Plotted a Perfectly Good Waste, as Ramiel has introduced another Prince of Hell.
The writers had this issue twice over with two of their despised female characters in the early seasons: Bela and Ruby. First, their core concepts as a non-hunter who acts as a human mercenary in the supernatural community and an Ambiguously Evil demon with hidden motives, respectively, were generally seen as interesting ideas handled badly due to Executive Meddling and writing and casting oversights, causing poor reception among viewers that led to both Bela and Ruby seemingly being written out by the end of their first season. (Bela was, Ruby came back for one more season.) Then as it turns out, their send-offs actually fixed a lot of the problems fans had with them, or at least presented even more interesting new ideas:
Bela was written as a more layered and more realistic Tragic Villain under the thumb of Big Bad Lilith and trying to find and assassinate the brothers to save herself in something of an Author's Saving Throw, before hellhounds killed her and dragged her to Hell. Judging by fan reaction, had she been written this way - a sympathetic antagonist Forced into Evil to try to save herself from a tragic Deal with the Devil - from the beginning, she would have been much better received than she was as the excessively smugRich Bitch she'd been who coincidentally crossed paths with Sam and Dean without explanation to make the protagonists look stupid and out-of-character and also flirt with them.
Ruby was getting less negative reception after awhile in Season 3 due to better writing than Bela tying her the myth arc as a major player and some fans warming up to her badass chick portrayal, making her departure at the end of the season a minor example of this trope. Her return with a new actress in Season 4 made her quite possibly the most disliked character at that point in the series, as she was writtenandacted differently than in the past season for both storyline purposes and as an acting choice by the cast, pissing off the fans all over again. However, The Reveal that she had been Lilith's double-agent working to free Lucifer through Sam, and her no longer needing to pretend to be on the Winchesters's side, opened many intriguing new possibilities to revitalize her character as a recurring villain, particularly her interactions with the Winchesters as a new demonic enemy and confirmed traitor. Her apparent feelings for Sam as her Morality Pet have potential as either part of her act to help solidify her as an antagonist who can flawlessly manipulate Sam and rub salt in his wounds, or genuine enough to be a possible route to redemption for her as a Well-Intentioned Extremist to ultimately once again become the traitor to Hell and Lucifer she previously claimed to be, to parallel Cas pulling a Heel–Face Turn for Dean against Heaven; either way, a brand-new direction to take Ruby in sure to cause lots of angst, anger, and Character Development in Sam, Dean, and possibly Ruby herself. Instead, she got killed by Dean mere moments after The Reveal, making her A Waste of a Perfectly Good Villain. In the Season 4 Companion Guide, Eric Kripke even admits that he had second thoughts about writing Ruby out during her last scene and realized it'd be more interesting to keep her in, but it was too late at that point to change it. This may be why he brought he brought another character he regretted writing out, Demon!Meg, back in a Ruby-type role.
Alton Morehead and Serena Colman are among the few British Men of Letters who don't really condescend to the American Hunters or come into conflict with them. They also get bullied around by Ketch The Dragon, and had a lot of promise, given their applying military logic to exterminating vampires with such devastating success. They could have stayed around to join Token Good Teammate Mick in providing a more reasonable bloc of the Men of Letters, but both die in their introductory episode.
Tyrus, the neutral angel leader who enjoyed Earth and made Metatron fume by telling him off and beating him at bowling, only gets one scene.
The two freed Sex Slaves/apprentice witches from Rowena's first episode could have stayed with her for a while in an interesting capacity but never even get a chance to learn magic.
The Head Angel, who has a truce moment with a demon in one of the funnier moments of early season 11, and attempts to lead Heaven against The Darkness, but is quickly and easily killed.
Elizabeth Higgins from the season 3 episode Malleus Maleficarium, the Token Good Teammate of a group of housewives tricked into selling their souls for with powers, who lasted a while into the climax and died casting a spell to help the brothers. Given her powers, somewhat interesting background from that episode and status as a likely damned soul struggling for redemption she could have made a good ally if she had been spared to be a recurring character.
The Living MacGuffin demon who was purified after possessing a man named Peter Kent then never seen again. Actually seeing a purified demon who could regret everything he’d done and wanted to atone as a presence on the show would have been impactful. For that matter, the very process of purifying demons itself deserved better than to just be attempted on Crowley once then never referenced again.
Atropos of the the Fates was introduced as an extremely powerful Pagan God who forces circumstances that kill lots of people, including ones that the brothers care about. Given her power, worldview and mutual It's Personal vibe with the Winchesters do you think she ever got made a strong recurring villain, or even seasonal Big Bad? Nope, she's never mentioned again after that episode.
There are many fans annoyed about how the Apocalypse World Hunters (especially Mauve Shirt Maggie) were so unceremoniously wiped out for the most part when their presence serving under the Winchester's had the potential to provide a more lasting impact.
Jim Jenkins from Death Takes a Holiday is a character who feels a little wasted in a single episode. A man who survived being shot in the head and was improving his life due to the absence of the local reaper, he could have ben a bigger presence in the episode, maybe interacting more with the brothers or helping them, possibly while realizing the potential implications it meant for himself and either trying to make peace with it or work harder to prevent his possible death (like through seeing a doctor to try and counter the wound head taken more). Instead, he's never seen again after taking to the Winchester's and even his fate is a bit ambiguous.
Muriel from Season 9's Holy Terror. After nearly every new Angel introduced in the series, especially those in Season 9, had thus far shown to be an unrepentant dick, she was a welcome change of pace being shown to be one of the very few reasonable Angelsnot to be horribly bloodthirsty and saw there wasn't any benefit to either side of the civil war going on, looking like a potential ally to Castiel and the brothers during a time when the three were quickly running out of them and desperately in need of a few more. Instead, she's abruptly killed off after just a few scenes in her one episode.
Hannah was a rare example of a sympathetic and relatively likable angel who struggled with many of the same issues Castiel had struggled with, and it was interesting to see Castiel have conversations with another angel that didn't end in violence. It's likely her crush on him doomed her, given the show's history with potential female love interests.
April Kelly certainly makes an impression in her one episode, and an evil, rogue reaper with a sexual history with Castiel could have been, at the very least, a recurring villain.
In the same episode Sarah returns, The adorably innocent Jenny and Tommy (the Taught by Experience first person the brothers ever saved), were also decent onetime characters who deserved better than to be back to be killed off by Crowley. It would have been nice seeing them band together with the brothers and Sarah for parts of an episode to actively resist his efforts to kill them.
Linda Tran was a fan favorite character, but she disappeared and was supposedly killed off screen after only four appearances. She did come back, but still in a pretty reduced role.
Emma, Dean's fast aging daughter with an Amazon, who was ordered to kill him as per Amazon tradition. Dean's only confirmed child, she could have been a way to explore his complicated feelings about fatherhood (especially after he spent a good chunk of the previous season acting as a father figure to Ben), as well as his attitude towards monsters after he killed Amy Pond. Emma herself could have made for an interesting character in her own right, as a Winchester who chooses humanity and free will over her monster instincts—she hesitates several times during the Amazon rituals, and initially seems sincere when she asks Dean for help escaping them. However, she turns out to be lying, Sam kills her to prevent her from killing Dean, and she is never mentioned again.
Abbadon in season 9, who despite initially being set up as one of the biggest threats of the season, ended up doing very little other than engaging in some Offscreen Villainy before being killed by Dean as little more than a stepping stone for the Mark of Cain plot. Despite this, many fans found her to be more interesting, more threatening, and better acted than primary villain Metatron, and felt she should have been the season's Big Bad instead.
Joshua is the Almighty Janitor of Heaven who comes across as their Only Sane Man, but then disappears for seven seasons and only returns to be pointlessly killed off with no further development or real plot importance right after taking charge of the remaining angels and seeming to be having them on a reasonable path based on his lieutenant Kelvin (who is also killed off despite being refreshingly sincere for an angel and developing a quick friendship with Castiel)'s interactions with Castiel.
Deliberately defied, however, with Castiel — originally, the writers planned for him to only hang around for six episodes and then be killed off. Then they realised that having an honest angel character who genuinely believes in God's plan who was caught between his angelic duties and his moral instinct, who was just beginning to get to know humans and feel emotions as he came to befriend Dean, was far too good a character idea to just throw away. And for good reason: Castiel has been a main character, and possibly the most popular character in the entire series, ever since.
Gavin MacLeod is a time-displaced Scottish peasant from the 17th century, the son of the King of Hell, and shown to be ambitious but a lot more sane about it than his father. He could have been the voice of reason, or at least someone sympathetic during the Crowley-Rowena dramas in seasons 10 and 11, but he's not even mentioned again until he's killed off in season 12.
