Lucille Lilliant from Gundam X, who's put into a forced coma and locked inside a capsule for 15 years. A whole arc is about her using the last remains of her powers to contact Jamil, her former pupil, through Tiffa's body so they can thwart the enemies' plans to capture her.
Yurin L'Ciel from Mobile Suit Gundam AGE. Due to her Psychic Powers, when the Veigans get a hold of her, they strap her to a MS cockpit and force her to become a living amplifier for Desil's own powers as he fights Flit, the boy that poor Yurin loves.
The anime Blue Seed has Momiji, a girl from a family line whose daughters, descended from the legendary Princess Kushinada, must ritually sacrifice themselves(or not) to seal the dreaded Orochi from rising again. Things were fine until twin girls (the aforementioned Momiji Fujimiya and her older twin Kaede Kunikida) were born, "splitting" the bloodline and rousing the demon. Of course, it doesn't help that Kaede, who is as powerful as Momiji but much more knowledgeable as well as very embittered, has come to the conclusion that Utopia Justifies the Means.
Ulrike and Ondine from Kyo Kara Maoh; being stuck in the same building for 800 years without cable would be torture.
In Magic Knight Rayearth this is part of the job description of the Pillar of Cephiro, and is the factor that drives the Backstory and underlying plot.
Konoka Konoe sorta was an unnoficial one in the Kyoto arc, which gets her kidnapped by a rival mage group who want to use her still not-awakened powers to unseal a massive demon. They fail and Konoka is rescued by Negi, Asuna and Setsuna.
Modern-day high-tech example: Chise from Saikano is the ultimate weapon on whose shoulders the fate of Japan depends...and the invasion is underway. When she tries to take time out for dating, lots of people die. Which doesn't even begin to cover it, in the end....
Saori Kido aka Athena from Saint Seiya frequently stood in this position. Notably in the Asgard (anime only) and Poseidon arcs, she uses her goddess' powers to delay Princess Hilda and Poseidon from flooding the planet and killing humanity. The effort was usually such a strain on her that it was a race against the clock for the saints to battle their enemies before time ran out and Saori died.
Also, Shun's Training from Hell and armor are heavily based on the legend of Princess Andromeda from the myths. You can guess what role he's filled more than once during battle... Lampshaded very often when his backstory is explained and everyone wonders if Shun's destiny is to die for others like Andromeda was willing to do. And he almost does once, for his friend Hyoga.
As Saori's expy and predecessor from the 18th Century, Sasha aka the Athena from Saint Seiya: The Lost Canvas is this as well. Before the rosary that seals the Specters' souls is created, Athena herself sustains a barrier that prevents them to be resurrected. She later manages to upgrade to Lady of War.
In Soul Eater the Shinigami sealed himself to the ground of what became Death City in order to imprison the first Kishin, Asura. 800 years later, Asura gets out thanks to Medusa's Magnificent Bitch credentials, but for the sake of the manga's storyline Shinigami stays put. The anime, however, finds a way around that. Also in the manga, Crona becomes the new guardian of Asura's seal. Unlike Shinigami, this seal is on the moon, so Crona has no one to keep company with in the meantime.
In Star Driver this is a main focal point for the plot. The reason that the Humongous Mecha don't show up in the middle of the island is because there are four maidens that contain seals to keep them in Zero Time; the bad guys want to break the seals so they can use the devastating weapons in reality. The show starts right after the villains have broken the first seal. THE BM here is Wako Agemaki, one of the Maidens, who gets trapped in a force-field bubble whenever the fights start. That's not being snarky; that is literally how it works. Poor Wako!
Kotori Monou from X/1999, who unbeknownst to her is one of the keys to the world's doom or salvation.
Kurogane's mother from Tsubasa Reservoir Chronicle was one of these. She was the miko to the local shrine which maintained the barriers keeping the demons at bay, and she was assassinated by Fei Wong Reed at a critical moment, resulting in the collapse of the barriers and the utter destruction of Kurogane's homeland, with himself as the Sole Survivor.
Narutos mother Kushina Uzumaki from Naruto. She was the former host of the Nine-Tailed Fox, and later the seal keeping him inside Naruto was made of her chakra. She's also the rare Barrier Maiden who mixes this is Action Girl, since she was in a Battle Couple with her husband Minato aka the fourth Hokage and Naruto's father.
If Kushina is one of these, then so is Mito Uzumaki, her Old Master as well as the first Kyuubi host.
A sort-of unofficial example is Anko Mitarashi. After defeating her in battle, Kabuto spares her...only to keep her prisoner and barely alive, to slowly absorb Anko's chakra so he can get all of Orochimaru's power, coming from her Cursed Seal.
The jewels inside the magical girls of Rakugo Tennyo Oyui are also what enforce the barrier around Edo. As far as barrier maidens go, though, they're pretty active in the field.
The title character in the manga version of Karin. Karin turns out to not just be a "blood creator", but a special kind of vampire named "Spirit of Psyche" that is born when the vampire species is struggling, so the blood Karin produces can bestow fertility in other vampires. Therefore, she's kidnapped by the Brownlick clan, which intends to first have the poor girl raped and forcibly impregnated so there will be more Psyche Vampires around, then feed from her blood to rejuvenate themselves — even when that's almost invariably going to kill her.
A certain character in Haruhi Suzumiya is a barrier for herself; if she gets in a bad mood, or discovers her own volatile abilities, she's likely to unintentionally erase the entire Universe (while not even being aware that she'll do so). No one is sure what would happen if she died, but she seems to have created the current universe three years ago and made herself its God, so it probably wouldn't be good.
In The Place Promised In Our Early Days, Sayuri ends up in a coma, which somehow prevents the giant tower from overwriting the world with part of a parallel universe. Destroying the tower might leave her in a coma forever, but waking her up could end the world (literally), so naturally they decide to do both.
Yuri Suzuki from Anatolia Story starts as this, since she's thrust into the Hitite Empire because Queen Nakia needs her blood to perform a curse she wants to cast on other kingdoms. She soon decides to fight back, however.
Played with in Blue Exorcist, where Ryūji "Bon" Suguro is a male example of this during the battle against the Impure King. His protector/companion Rin knows his tropes, and mockingly calls him "Princess" for the duration of the fight.
The king of the Catacombs in Undertaker Riddle. As holder of the Twilight's key, an inmense power able to created or destroy worlds, the king use it to protect the Catacombs of soul between the human world and the afterlife. Faust is the current king while Hayato is her expected succesor.
Sayaka Mine from Yaiba. More exactly, she's a descendant of Taro Urashima and Otohime: that marks her as the "Dragon Maiden", the Priestess of Ryujin and the key to unleash Princess Kaguya's true form. And uncommonly for this kind of story, Kaguya does manage not only to capture poor Sayaka, but to absorb her Life Energy to unlock her big form; Sayaka survives but he body is drained to the point of rapid aging, and she doesn't recover until Yaiba defeats Kaguya.
An issue of the Buffy the Vampire Slayer, Season Eight comic features a young woman performing this task for a demonic house and not feeling at all well thereby. The character was based on a mentally ill fan, who evidently liked it.
Fantastic Five, a what-if future continuity of the Fantastic Four, sets Sue Richards up as one in a quite literal sense. It's used as a surprise twist: A massive rift in space-time was just about to tear a few dimensions wide open. Sue was critically injured trying to close it. So, as her final request, Sue's comatose body is suspended in a massive generating machine, on a space platform in interdimensional space, so her force field powers can keep the rift from breaking loose and destroying the universe. Reed stays with her in a lonely vigil, continuing his work for the Five on Earth through remote-controlled proxy robots that the group claims are cyborgs.
In Shadowchasers, Tharizdun is a villainous example. The gods dare not kill him, despite the potential genocidal cataclysm he would attempt to start if he was freed from his prison; killing him would allow [[Cthulhu Mythos the Great Old Ones to reclaim the power they invested when they turned him into their Unwitting Pawn, potentially resulting in them releasing a far more dangerous threat.]]
The protagonist of The Sentinel (not the television show) becomes one such at the end of the movie, guarding the literal Gates of Hell (she partially qualified by being a failed suicide).
The Childlike Empress in The Neverending Story was a Fisher King for the land and her illness is thought to be what's causing The Nothing that's destroying Fantasia, but her sickness is actually due to her needing a new name. (Fantasia is failing because very few humans are visiting these days. The two problems are related and can be solved in a similar manner.)
Ehlana, Queen of Elenia in the Elenium trilogy by David Eddings, serves as a Barrier Maiden during the first 2 1/4 books. The ghost of a dead knight comments to the series protagonist that "the darkness hovers at the gate, and Ehlana is our only hope of light." The action of the series mostly focuses around recovering the mystic artifact that will keep the poisoned queen from dying, and thus prevent the world from succumbing to the grip of evil.
Unusual in that it's not any magical power she has that makes saving her critical, but that allowing the next in line (her cousin and suspected half-brother) to take the throne would give the villains enough cash to buy leadership of the church and turn it into a Church Of Evil.
In The Elfstones of Shannara, one of the main characters is a Barrier Maiden. A bit subverted, in that a magical tree is keeping shut a rift to the Demonic Underworld, and it begins to die. The female lead, Amberle, is the chosen one, but we only find out in the climax that this doesn't mean she heals the tree - she becomes the tree. Which is a bit sad when you're a 12-year-old kid.
While the Archive from The Dresden Files doesn't actually prevent the destruction of the world by her existence, she is a repository of all mankind's knowledge and knows everything that is ever written down. Particularly in Small Favor, she's treated as quite possibly the single most badassBarrier Maiden in all of history. There is also the Oblivion War, which is a battle against ancient demonic gods who still want to rule this reality. The participants in it are few, and rightfully so. They seek out to destroy all instances of a god's name on Earth, including in human memory. Once it is purged and those who fought to purge the name forget themselves, Archive deletes the name from herself and banishes the god from Earth forever.
Cold Days reveals that Mab protects reality from the Outsiders, and Titania protects everyone else from Mab.
A rather darker example of the trope: in The Vampire Chronicles, the first vampire, Akasha, is in suspended animation and closely guarded in an underground vault by her vampire servants, since, while she herself is virtually indestructible, if she is harmed, all the vampires dependent on her would be destroyed. In ancient times, a usurper places her in the sunlight: while she only receives a tan, vampires around the globe burst into flame.
There is a clear example in Tolkien's The Silmarillion: Melian the Maia uses her powers to guard and defend Doriath with a protection called the Girdle of Melian.
Galadriel does the same with Lothlórien in The Lord of the Rings. It should be noted that it was Melian who taught Galadriel her stuff.
To a lesser extent, Lúthien (who happens to be Melian's daughter): the War of the Jewels is temporarily suspended as long as she's wearing a Silmaril, because she's Lúthien and nobody wants to take the jewel from her. As soon as it passes to her son Dior, the war starts right back up again.
It helps that she's raised an army of cultists around herself in the Old World, and intends to bring them to Everworld, where she will use their 20th century tech to overcome the gods.note To clarify, technology is shown to be quite effective against the denizens of Everworld, with several mythical beings getting gunned down easily.
The Eternals from Fablehaven are a group of immortal Barrier People (not all maidens) who serve as the second lock to opening the demon prison Zyzyx. Immortality has notbeen kind to them, such that it is not actually all that difficult for them to be convinced to take their own lives. The Big Bad's Ironic Hell is to become one of them.
Princess Jenna from Septimus Heap. Basically, the safety of the Castle depends upon her presence in the Castle, leading to both the plots of Magyk and Flyte as she is persecuted by DomDaniel and actually kidnapped respectively.
Jelena of Griffin's Daughter is of the Cosmic Keystone variety. The Key, a source of immense magic power, was sealed within her while still in the womb, by elf mages who pulled her mother through time for the purpose.
Live Action TV
Dawn of Buffy the Vampire Slayer is the Key that holds closed the barriers between dimensions. Season Five centers around a Sealed Evil in a Can trying to leave Earth by using Dawn. Not only that, but near the end the heroes get into a discussion of what to do if the worst happens, and we get to watch Giles and Buffy argue over whether killing Dawn is an acceptable last-resort option...with Buffy finally stating flat out that if any of her friends try to harm Dawn, she'll kill them. Turns out she actually feels Dawn is part of her, and closer to her than Willow, Xander, or anybody...and she ain't doing the Angel-skewering thing again, not even to save the world.
Two seasons later, as the series finale approaches, she and Giles discuss the matter again (hypothetically) and she admits that her decision at the time as based on emotion not reason or morality. If she had to make the same choice again in the present (one life to save the world), she's not proud to say she might choose differently.
Lunagel (Rin) from Mahou Sentai Magiranger is the gatekeeper whose body and life keeps the evil monsters imprisoned in Infershia. Clare serves as her US counterpart in Power Rangers Mystic Force, though unlike previous gatekeepers, she nonsensically gets to stay alive when the gate is inevitably broken.
A bit different in both versions. In Magiranger, her life force maintains the seal, so if she is killed, the gate will open. (The wheel thing above her head while she was captive, there in both versions, was to break the protection spell that normally makes this impossible.) In Mystic Force, the title of Gatekeeper is passed down, but fully sealing the gate killed Clare's mother, so it's said that opening it will kill Clare. The wheel thing was designed to tap that power. (She should still have died, though, and there wasn't even a Hand Wave. Rin, on the other hand, is in the clear, as it's not operating the gate that would kill her, but the other way around.)
Princess Astra in the Doctor Who serial "The Armageddon Factor."
The Keeper of Traken from the serial of the same name is quite possibly the inspiration for the Magic Knight Rayearth example. The Keeper has great power, but having to focus that power to maintain the Traken Union has to get And I Must Scream-y after having to do it and nothing else for years and years and years, but that's not the plot point that it is in Rayearth. Also, like Rayearth, there are still those who want to take the position despite knowing this.
In Supernatural we get Lilith, the devil's right-hand man — er, woman, I mean, demon — who is the final seal holding Lucifer in his cell. Once she's dead, he's free to bring the apocalypse. Interestingly, there's a completely separate (male) character serving as the Apocalypse Maiden.
The Imagin of Kamen Rider Den-O want to make their Alternate Timeline a reality by destroying the existing timeline. They travel to the past, laying destruction there, but the existence of Singularity Points (the Kamen Riders) prevent any permanent changes. The Big Bad then figures out that a special Singularity Point, called the "Junction Point", is the one keeping the timeline stable, and that the Imagins' victory will only be ensured by destroying the Junction Point.
In the Synchronicity series of Vocaloid songs, the Dragon Diva is a girl who must keep singing and dancing to appease the dragon who keeps the land in a perpetual state of prosperity. Once she becomes the Diva, she is cut off from the rest of the world until she dies, at which point a new Diva is chosen to take her place. The story focuses on Len's quest to rescue his long-lost twin sister Rin, who was kidnapped as a baby in order to become the current Diva, and stop the cycle once and for all, even if it guarantees the nation's eventual decline and ruin.
An example from the Christian martyrology: the unnamed Princess from Saint George of Cappadocia's legend, as well as her Japanese counterpart, Princess Kushinada from the Orochi myths.
Ashling from the Magic: The Gathering setting of Lorwyn was supposed to be one of these, bringing the flamekin's fire through the transition to Shadowmoor. She didn't go along with the idea.
The God-Emperor of Mankind from Warhammer 40,000 can be considered an example. His psychic might maintains the Astronomican, the only thing keeping the armies of the Imperium capable of navigating through the Warp. If he ever dies, the Astronomican vanishes and the Imperium becomes a million worlds alone in the dark, waiting for death.
Warhammer has Caledor and the other High Elf wizards locked upon the Isle of the Dead. They are time-locked in a constant ritual that keeps the power of Chaos from overwhelming the entire world. If they ever die, or are ever freed, it's the end of everything.
....and before her came all the peoples of Creation, and knelt.
They set aside their petty squabbles in her name.
They acclaimed her with many shouts.
They prayed for her safety, and her weal.
Not one of them struck free her chains.
"To know the world..."
"...is to own it," she said.
In Kingdom Hearts The Seven Princesses of Heart collectively have the power to reveal the keyhole necessary to get to Kingdom Hearts, the origin of life in the universe and a recurring MacGuffin. The bad guys do all they can to find the missing Princesses so they can fully take over.
On a bigger scale, the Princesses' hearts of pure light repel darkness and keep all the worlds in the Realm of Light safe just by being there.
In the World of Mana series, The World Tree is not the same Mana Goddess that created the world as legend says, but is a person of some variety who became the new Mana Goddess/Tree the last time the world needed a new one. This is played to particularly devastating effect in Sword of Mana, the remake of the first game, in which the female protagonist plays this role. It's particularly devastating in Sumo's route since she's also the Love Interest of the main character there.
The Legend of Zelda: A Link to the Past has seven of these (successors to the seven Sages in Ocarina of Time) maintaining the seal on the Dark World. Princess Zelda also takes on this role voluntarily in The Legend of Zelda: Twilight Princess, first when she surrenders to the Twilight King Zant in order to save her people from being massacred, and again later when she transfers her powers into Midna, to stop the imp from dying, and appears to die herself in the process.
Likewise, Dragon Quest VIII featured seven descendants of the ancient seven sages who sealed Rhapthorne in the Godbird Sceptre, and who Rhapthorne had to kill (using others as pawns) to restore his corporeal form. The fate of these descendants is obvious.
In StarCraftKerrigan is revealed to the only thing holding back the void from destroying the universe.
The Barrier Maiden thing was abandoned for a while, with Bowser's Motives changing from pure conquest to trying to win Peach's affection. However, more recent games have restored her Barrier Maiden status, though in a different form each time.
In Yoshi's Island DS, Baby Peach is one of the Seven Star Children who are born with great power. Bowser attempts to steal the children to take over the universe (this would include Kidnapping himself, which would result in a few time related issues...) Peach is actually one of the children who escapes capture, and assists Yoshi and Baby Mario in saving the others.
In Mario & Luigi: Superstar Saga, Peach's voice is the only thing that can awaken the Beanstar, which grants wishes. She avoids capture through clever use of a Birdo, and again by having Luigi crossdress as her.
In Super Paper Mario, Peach and Bowser are a Barrier Couple. If they ever got married, the Chaos Heart would be formed and begin to destroy the world. Naturally, this is the first thing the Big Bad does in the first part of the game. Peach is so easily kidnapped...
Fatal Frame. It's eventually revealed that Barrier Maidens were used to ritually seal the Hell Mouth under the house. You find out that all hell broke loose because the barrier maiden had only just caught sight of another man at some point before the ritual.
Fatal Frame 2: The Barrier Maidens are twin girls (or twin boys). One of them is sacrificed to hell...darkness...abyss thing to prevent it from devouring the village, while the other stay in the village to act as a living goddess of some sort. The real Squick part? The girl who is to remain alive must strangle her sister as an act of sacrifice. And that's just the beginning of the circle of tragedy that engulf a lot of people, including the protagonist Mio and her handicapped twin sister Mayu.
Colette from Tales of Symphonia, the Chosen One, intended to sacrifice herself to keep the evil of the Desians sealed away. Really suffers for it, both for the cost of performing her mission, as well as most of the villains and even a few others treating her as a MacGuffin. Not to mention her mission is a complete lie told by the Desians themselves, who were never "sealed away," her actual purpose being to save her world by screwing over another one, and becoming a soulless vessel for the Big Bad's dead sister.
Richea Spodune of Tales of Hearts is the non-sacrificial version, and a double serving. First, she's the only thing holding The Heartless in check. When she's temporarily incapacitated, the Zerom and their symptomatic Despir Sickness return to the world like a sledgehammer. Second, the Big Bad requires her power to activate the Space Whale to go release a world-eating monster (he has a reason, trust me). Her elder sister is also one, having fused herself to said monster to seal it away in the first place. Yet another variant example is a massive Power Crystal powering a barrier protecting the village of Norquin. The maiden here is dead, but it's said that her will to protect the village remains in the Psistone and powers the barrier.
He was not only just holding back Nyx, but also Erebus, who is the Anthropomorphic Personification of humanity's anguish and wishes for death. If Erebus and Nyx made contact, then the world would end. The protagonist becomes a wall (sealed in stone and stuck to it, no less), destined to stand between the two cosmic horrors forever, or at least until humanity as a whole is changed and stops subconsciously wishing for death. This could mean that he's there to stay but his former companions resolve to do what they can to change the world. Considering three of them could live indefinitely - Aigis being a robot, and Elizabeth being...whatever Elizabeth is - it could be possible in the long run.
The PSP portable version adds a female protagonist. Despite how she and the male MC aren't exactly the same characterization-wise, she fulfills the same role in-story — and it includes becoming the local Barrier Maiden, too. For better or worse, she can fall in love with the Appriser of Nyx, Ryouji Mochizuki, which adds a Star-Crossed Lovers element here. And here we have three possible companions that could potentially survive until the day she's released: Aigis, Elizabeth, and Elizabeth's younger brother Theodore.
Another spin of the Kushinada myth is used in The King of Fighters 97. First, like in the original legend there's a group of girls that are kidnapped, and all but one of them become Human Sacrifices for Yamata-no-Orochi; instead of having Susano-o save and then marry the Sole Survivor aka Princess Kushinada, she's rescued by the leaders of the Three Sacred Clans, who then seal up Orochi since they can't kill him. Second, in the present Kyo Kusanagi's Tsundere girlfriend Yuki is the last descendant of the remaining Kushinada, so the Orochi clan abducts her and plans to sacrifice her in the ritual that will bring Yamata no Orochi back into this world. She survives her ordeal thanks to Kyo, Chizuru and Iori, though.
Another character from the same series, the Idol Singer Athena Asamiya, is a rare case where the Barrier Maiden is also an Action Girl. As an Expy of the already mentioned Saori Kido, this is expected.
And then we have the Kagura sisters, Maki and the aforementioned Chizuru, two Mikos who keep the Seal of Orochi safe and tight so he won't return to unleash destruction on Earth. When Maki was murdered by Goenitz, Chizuru was left alone with the mission of taking care of the seal, so she recruits Iori and Kyo in KOF 96 to do this and then helps them re-seal the released Orochi in KOF 97.
Another girl named Yuki plays the BM role in The Last Blade. Unlike KOF's Yuki, when offered the chose to either seal the Hell's Gate or not, TLB!Yuki ultimately decides to do so at the cost of her own life.
Also in this category, in World of Warcraft, is Anveena Teague, who is the embodiment of the remaining energies of the mystical Sunwell. Eventually, she sacrifices herself in order to help the players defeat Kil'Jaeden, and perhaps restore the Sunwell.
As of Wrath of the Lich King, we can add Bolvar Fordragon to the list. Essentially, Azeroth needs a Lich King to keep the scourge from going all zombie apocalypse on the planet. Bolvar decides, after Arthas's defeat, to succeed him as such, and essentially act as a "jailor of the damned" from the Frozen Throne.
Final Fantasy VII has a VERY rare Inverted-Evil example. Sephiroth has the good locked away, and you must kill him to release it.
Summoners in Final Fantasy X would qualify except for Yuna, of course, who breaks the cycle.
A villainous version also appears in Final Fantasy X in Yunalesca. She tells the heroes (and every group that had made it that far for the past thousand years) that not only must the Summoner sacrifice herself to save the world, but one of her guardians. Then, in order to leave Zanarkand, fight Sin, and save the world for real, she must be killed.
Shana, from The Legend of Dragoon, is an evil example. At first, you think of her just as The Chick and The Medic, but then you discover she is the Moon Child, the only being able of resurrecting the Virage Embryo, last boss of the game. Until she is used by Melbu Frahma for this purpose, no one, not even her, is aware of her real identity.
Misha, one of the three potential love interests in Ar tonelico.
Morrowind gives us the Tribunal, specifically Vivec. By the time the game takes place, he has been maintaining the Ghost Fence on his own for several centuries, without being able to replenish his divinity. It is the only thing holding DagothUr and the Blight at bay.
Another rare male example: Martin Septim in Oblivion, like the other Emperors before him, is the only person who can carry out the ritual that seals the barrier against Oblivion. At the end, Martin sacrifices himself, sealing the barrier forever.
In Skyrim, the Thalmor have made worshipping Talos illegal. Why? according to the lore, Talos is one of these, and if nobody worships him, along with the Heart of Lorkhan gone and the Dragonfires extinguished, Tamriel itself will cease existing.
Sakuya from Ōkami is a cherry tree spirit and powerful protector. Her guardian saplings protect the nearby lands from evil, and their corruption not only weakened her into a coma but allowed evil to flourish. Amaterasu's missions largely revolve around restoring her saplings, culminating in Sakuya being restored and 'refreshed'.
Dragon Quest VII has a particularly cruel version, with the warrior Matilda, as she's being used by the monsters as a Barrier Maiden sealing away their hostages. Making matters worse is that she's a Broken Bird due to her past: her brother tried attacking the monsters' stronghold, but the other villagers chickened out, leaving him to fight until his death waiting for the promised backup that never arrives.
In the Girls LoveVisual NovelAkai Ito, this is the job of whoever is taking the role of Ohashira. The first Ohashira was Otsune, and in the present day it's Yumei. The Ohashira essentially become a ghostto seal the soul of Nushi with all the might of her soul. It's quite similar to the myth of Sakuya mentioned above. In fact, a character named Sakuya was really attached to Otsune.
Nostalgia has one in Fiona. She is the only one who can touch the tablet fragments, thus preventing the bad guys from collecting them without her. She also has to die to seal the Big Bad away.
It was in Mega Man Zero 2 did we finally find out what happened to the hero of the previous series. He used his physical body to seal the Dark Elf, a powerful Energy Being that is the cause of the World Sundering between the X and Zero series. This leaves X in borrowed time, only appearing to Zero as an Energy Being himself. Destroying X's physical body would release the Dark Elf, which would mean another mass genocide if she's in the wrong hands. And it just so happens to be the goal of the Big Bad of Zero 2.
In Dark Souls, Gwyn, Lord of Sunlight, is this by virtue of being the Lord of Sunlight. In actuality, his real purpose as a Barrier Maiden is that he sacrificed himself by Linking the Fire, turning him into the fuel that allows all fire in the world to continue existing. Whether or not that is a good thing is an entirely different matter.
Hazuki Kazama from Samurai Shodown is a girl who used her powerful sealing techniques to tame and seal two inmensely strong spirits, Enja and Suija, in the swords of her older brothers Kazuki and Sougetsu. Too bad that Amakusa heard about Hazuki's abilities and took a hold of her for the worst...
In Breath of Fire II, a giant dragon,who is Ryu's mother, is seen blocking the door that leads to the final dungeon to keep the Big Bad from coming out. In a normal ending, Ryu replaces her since she dies.
In Prince of Persia: The Two Thrones, Kaileena serves as this until the middle of the second cutscene, during which a) it is revealed that her continued presence on her island was the only thing keeping time linear and b) her death allows the (once again alive due to time travel) vizier to ascend to godhood and plunge Babylon into darkness.
Deirdre from Fire Emblem: Genealogy of the Holy War is this due to her heritage. On one hand, she's the long-lost daughter of Prince Kurth and carrier of Naga's bloodline. One the other, she has minor Loptyr blood, making her a target for his cult's plan to resurrect him, so they abduct her and get her to unknowingly marry her half-brother.
Snowman from Homestuck is an evil Barrier Maiden—no one will kill her because they know if they do the whole universe will end, and she takes advantage of this to torment Spades Slick without fear of retribution. She dies anyway.
Old Woman Josie seems to be this to Night Vale, telling Cecil rather ominously that "if she falls, so does this town".
Will Vandom and the other Guardians from W.I.T.C.H. may be partial examples, as the Veil that separates the worlds is drawn from the Heart of Candracar and the resulting drain on their power makes them weaker during Season 1, Will having the worst of it since it deprives her of any offensive ability; she doesn't even know what her elemental affiliation is! When the Veil is lowered in Season 2 all the Guardians receive a power boost as their powers return, Will gaining the power of Quintessence which is Shock and Awe, the ability to animate objects and give personalities to electrical appliance among other abilities.