The Morality Chain is a character who is the reason another character is Good. Stereotypically a female love interest, a mother, a daughter, or a little sister; as long as this person is alive, her target of affection will at the very least be a Noble Demon.
If a Morality Chain were to fall, commit betrayal, or get seriously hurt or die, there is nothing to prevent a Face Heel Turn happening so fast and so hard that the unchained character is gladly chopping his former teammates in pieces before you can say "Neutral Evil". This is more than the Roaring Rampage of Revenge; everyone has to suffer.
The Morality Chain is the only thing preventing someone from happily killing their "friends" and family. One wonders whether such a form of "goodness" has much value in it (aside from, well, keeping them from killing their friends and family).
A very similar, and often confused with, trope is the Morality Pet. The difference between the two is subtle: a morality pet is a character who redeems a villain. The villain's affection for the pet starts them down the path of good, and even should the pet get hurt the villain will most likely behave as a hero (or at least an anti-hero) in seeking their revenge or protecting the pet. By contrast, the morality chain is the only thing that keeps an otherwise anti-heroic character (such as a Sociopathic Hero) from going full villain. The loss of the chain would spell doom for any involved party, and likely anyone nearby as well. Think of it this way: a morality pet turns a bad guy good; a morality chain stops a good guy from turning bad.
Sometimes this is inverted. The death of the Morality Chain motivates the target of affection to become more determined to be good. To honor her memory, or something. Usually in these cases the cause of death is either natural, or because of a villain (especially if it's a buddy of theirs that the Morality Chain had disapproved of). Now, if the cause of death is their loved one, either through an accident or because they Kicked The Morality Pet, then they may very well either go comatose or just plain crazy.
See also: Morality Pet, Cynicism Catalyst, Morality Chip, Living Emotional Crutch, Driven to Villainy. Contrast: The Kid with the Leash, The Farmer And The Viper.
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Anime and Manga
In Berserk, Casca is this to Guts. Being Guts's lover driven insane by the actions of Griffith and the Godhand during the Eclipse, she is the only reason why he refuses to completely give into his Super-Powered Evil Side. Said evil side, a Hell Hound-like beast, is fully aware of this and constantly importunes Guts to kill her so it can take over his mind and turn him into a being of pure hatred. The Beast is literally chained thanks to Gut's devotion to Casca, though it promises Guts that it will be free one day.
It can be argued this applies to Sherry Belmont's precious Koko in Zatch Bell, considering it's implied without Sherry protecting Koko's fragile innocent self, terrible things would start to happen. Such as Koko getting controlled by an Evil demon allowing her to burn down her entire village, and then running off from Sherry's grasp. Of course it ends with (a restored) Koko returning to Sherry so she can be protected and not grow to be a terrible person, or really anything Sherry isn't comfortable with. (Which is pretty much to Sherry's hope nothing at all worth mentioning.)
Kara no Kyoukai plays it straight. The only reason Shiki doesn't go around killing people is because she understands that she would break Mikiya's heart if she betrayed his trust. It's only after she's (incorrectly) told of his death that she decides to kill the one responsible. He's diced into half a dozen pieces before he even hits the ground.
In Yu-Gi-Oh!, Malik's Superpowered Evil Side takes over after his loyal servant Rishid is knocked unconscious. Later, when Rishid wakes from his coma, he is able to persuade the real Malik to overcome his evil side.
Yugi and his True Companions are this for Yami Yugi. They have stopped Yami from injuring or killing others several times.
Yugi especially, seeing how the very same day Yugi's soul is taken, Yami Yugi loses his cool after Weevil tears up a card in front of him claiming it was Yugi's soul.
Nunnally in Code Geass is Lelouch's Morality Chain. Lelouch's main motivation comes from terrorists assassinating his mother and crippling his sister, which led his father (the emperor) to abandon Lelouch and Nunnally. Because of this, he's motivated to avenge his mother, destroy his father's empire, and build a gentler world where his sister can be happy. Even with Nunnally alive, Lelouch already skirts the edges of Villain Protagonist...
When Lelouch's own actions indirectly caused the loss of his Morality Chain, Lelouch vowed to personally drag his father to Hell with him. When he found her again, she had learned his motives. And she did NOT like being the Morality Chain.
Nunnally didn't even realise that she was the Morality Chain. She just thought he was downright evil for killing Euphemia, and who knows who else, thanks to Schneizel's interpretation of events... and Lelouch's own preventions against his (correct) interpretation of Schneizel's character.
Lelouch wasn't completely out of it until the Black Knights' sudden betrayal pushed him entirely past the Despair Event Horizon. And the "drag his father to Hell" bit was just a bit of a fancy Taking You with Me proclamation.
In Fate/stay night, Ilya is evidently this to Berserker. Note, he's still a massive lump of destructive impulse given terrifying form, he's just better tempered when she's around.
Rurouni Kenshin features this for everyone's favourite Hitokiri; Kaoru's presence during Kenshin's duels with everyone prior to Saitou was what kept him from falling all the way back into the ways of the Hitokiri. Yahiko also briefly functioned as this during Kenshin's Curb-Stomp Battle with Raijuta; he told Raijuta that the fact that he was still alive (ie; Raijuta hadn't killed Yahiko) was the only thing stopping Kenshin from murdering the man.
Prior to the Kenshin-gumi, his first wife Tomoe Yukishiro's presence makes Battousai less of a cold, ruthless killer. As his employer, Choshu clan head Katsura Kogoro, asked of Tomoe, she serves as "the sheath to temper his mad justice."
In Haruhi Suzumiya, an important point is narrator Kyon's role as this for Haruhi. Before Kyon, she was a sociopathic, melancholic jerkass and with him, she's a sociopathic but cheerfulJerk with a Heart of Gold. Kyon also is the only one who can hammer some sense of right and wrong into her. It is a very credible possibility that, should Kyon ever die, the universe would be destroyed. Ryoko Asakura tried to prove this theory right or wrong by trying to kill Kyon herself to see what would happen to Haruhi, but is stopped in a Crowning Moment of Awesome by Yuki Nagato.
In the final Slayers light novel arc (currently Japanese-only), Milina serves this role for Luke. When she dies, Luke's anger and despair trigger the Second Resurrection of the Dark Lord, causing him to become Luke-Shabranigdu.
From what little we've seen of Stein recently, Marie's influence now Medusa's snake is gone seems to be doing the trick. His reaction to Marie after BJ's death showed that in some way he really does care.
Spirit may have something of this, although his role seems to be to keep Stein in line - he states during the Medusa fight it's his "job to control this crazy kid". However, he does seem genuinely concerned about Stein's welfare and is the one who allows him to 'escape' and search for BJ's real killer.
In Black Lagoon, Rock acts as a morality chain for Revy that moves her from Neutral Evil to Sociopathic Hero, willing to help Rock pull off good endings in the Yakuza and El Baile De La Muerte arcs (failure and partial success respectively). Suffice it to say that in the event of Rock's death, Revy would make the events of the Nazi arc look like cheerful shoujo by comparison.
Let's not forget that the Lovelace family is this to Roberta. Her Roaring Rampage of Revenge in El Baile de la Muerte started when Garcia's father, the leader of the Lovelaces, was blown to smithereens.
It's kind of scary to think of given how unbalanced the cast is already, but several episodes of Sayonara, Zetsubou-Sensei indicate that Itoshiki-sensei fills this role for his class. Without him, their loneliness and mental instability would drive them completely over the edge.
In Bamboo Blade, Miyako "Miya-Miya" Miyazaki has her boyfriend Danjuro Eiga as her morality chain. It's fairly evident throughout the series that she only cares to keep her repressed aggression in check whenever he's around.
Umineko No Naku Koro Ni has Hideyoshi Ushiromiya fulfilling this role for his wife Eva. Whenever he kicks the bucket (which happens in every arc), shesnaps.
And Luca who was Alois/Jim's little brother was implied to be this for Alois/Jim. Alois was very protective of Luca, wished death on people who hurt Luca, and when Luca died Alois fell into despair, became very ruthless and quickly swore revenge on whomever he suspected to have had a hand in Luca's death.
Yachiru from Bleach is this to Kenpachi, to an extent, even though she can be just as violent as him at times. Kenpachi may be a savage now, but before he took in Yachiru he was little more than an animal with a sword. All he did was kill; he didn't even care that he had no name. He may be on the bottom of the morality ladder now, but before meeting her, he wasn't even aware it existed.
Possibly you could state that on the blimp, Gon played this part for Killua. In the anime, he was about to kill a girl named Anita when he heard Gon still fighting for her outside, and it brought him back to his old self; since then, he's actually seemed to avoid killing people if there was an alternative.
Wolf Guy - Wolfen Crest: Aoshika-sensei "graduates" from Morality Pet to this, for Inugami. But in an horrifying twist, once Haguro realizes it... he actively seeks out to break her completely so Inugami will go back to him. Even by brutally raping and torturing her, if needed.
Fushigi Yuugi: With Amiboshi around, Suboshi was as much a Jerk Ass. When Suboshi thought he was dead... all he needed was a push from Nakago to fly off the deep end and kill Tamahome's family in revenge.
In One Piece, in some ways, Fisher Tiger was this to Arlong and some of the other crew.
Koala could be argued to be this for Tiger since her innocence convinced him that there was a hope for positive relations between fishmen and humans, an ideal that he finally believed on his death bed.
Naruto: Itachi's existence notably keeps Sasuke's sanity anchored to this world. Back when Itachi was alive, Sasuke may have been angsty, but not insane. After he killed Itachi he began to lose it, but, for the short period of time that Itachi is "alive" again, Sasuke was acting far more sane than he has in 100 chapters. When Itachi dies again (this time explaining things more thoroughly to Sasuke) Sasuke is bordering back onto his insanity cliff again half determined to destroy the village and half unsure as to what anything means.
Rin certainly is an interesting case. She is basically the one person that glues together her team, and when she dies, the whole team completely falls apart. Obito becomes a masked terrorist under the alias of Tobi, Kakashi becomes a pathetic self-loathing man wracked with chronic guilt depression. Had she not died, both men would not have become the broken men they are now.
Shion is this to Nezumi in No. 6 with all the Ho Yay implications that come with it.
Gunslinger Girl plays this oddly with idealistic former Europol officer Victor Hillshire, where Triela, the girl he rescued from the set of a snuff film and threw away his career in the hopes of getting her rehabilitated is his chain to an Italian wetwork squadnote Brainwashed Kill-Botdoes count as rehabilitated if you stretch things a little. He would have thrown his life away rather than cooperate with the Social Welfare Agency, but who would take care of her?
Sagara Yoshiharu is this towards Oda Nobuna in Oda Nobuna No Yabou. Being forced to play the part of Toyotomi Hideyoshi (who incidentally, was killed saving Yoshiharu) to this timeline's Oda Nobunaga (who also turns out to be a girl) during the Sengoku Period in Japan, he changes a lot of things around largely by pleading to her merciful side, such as forgiving her brother for his small rebellion rather than kill him as the real Nobunaga did. When Yoshiharu is presumed killed later in the series, she goes on a Roaring Rampage of Revenge, which puts fears to her rivals, but concerns her allies that she may be crossing a Moral Event Horizon.
The Saint of Killers' family in Preacher - it is after their death that he becomes so full of hate, which in turn leads to him freezing Hell after his own death and being transformed by the Angel of Death. It is later revealed that the death of his family was orchestrated by God to turn him into the Saint of Killers, deliberately freeing him of his morality chain.
Lampshaded in Runaways when Chase reveals that he believes his girlfriend Gert is the only thing keeping him in line. He does go off the deep end for a while after Gert's murder, but he never really becomes evil.
In Young Avengers, we saw how quickly Gentle Giant Hulkling lost his shit when his boyfriend (who is ironically the grandson of a supervillain) sort of disappeared from reality.He got better when said boyfriend was back.
Dr. Manhattan from Watchmen doesn't have a Morality Chain so much as a Humanity Chain, in the form of his girlfriend Laurie (also known as Silk Spectre). He states openly that the only reason he maintains any interest in the earth is because of his link to her, and when she leaves him he heads off to Mars within the day, completely indifferent to the fact that his leaving is likely to trigger nuclear holocaust.
Notably, it's also their later conversation and partial reconciliation that convinces him to return.
Isis serves this role for Black Adam in 52. After she dies and comes back to life, he tries to dissuade her from indulging in her new omnicidal mania, meaning he's trying to be her morality chain.
The comic also has Superman acting as this to Lex Luthor of all people. Granted, Lex is pretty damn evil when Superman is around, but when Superman drops off the face of the Earth and Lex doesn't have him to obsess over, he creates the Everyman Project, which can turn normal people into superheroes, in order to test the process out before he uses it on himself. However, when he discovers he's incompatible with the process, he angrily shuts off the superpowers of everybody in the project, standing on the top floor of LexCorp as he watches the dozens of Everymen that were flying at the time plummeting to their deaths.
Deathstroke's Battle Butler Wintergreen was this for the entire Wilson family. His Undying Loyalty made him complicit in Slade's shadier dealings, but he always tried to bring out the best in him and his children. Wintergeen's death at the hands of the Jericho-possessed Deathstroke destroyed any chance that Slade could ever go through a true Heel Face Turn. Rose at least managed to eventually turn good.
In Justice Society of America, after Damage dies, Judomaster goes after her father's killer because of the loss. Sand reveals to her that Damage, forewarned of his death, had left her a message; this is what persuades her not to kill him.
Fluttershy becomes this for Rainbow Dash in Rainbow Double Dashs Lunaverse, as Rainbow Dash in this continuity never became the Element of Loyalty, thus never forming the True Companions like in the main story. This also deconstructs this trope in that with just a Morality Chain and no one else, she is unable to extend that sympathy to other people or really relate to them.
In the Avatar fanfic by the same name, Zuko (and by some extent Ursa) is a morality chain for Azula. Striving to earn her mother's affection, she goes along with her promise to watch over Zuko. She is still a manipulative psychopath with little concept on base emotions like love and still out to capture the Avatar, but she shows genuine care and concern for not only her brother but her friends Ty Lee and Mai as well. You cannot help but to root for her in some scenes.
Four Brothers has Evelyn Mercer serve a similar function, as the following quote demonstrates. That said, as soon as she dies it's as if Sauron were let loose in that town. Eventually, the brothers do somewhat settle back down once enough people are dead.
Detective Fowler: If this woman's such a goddamned saint, how did she end up raising four total fuck-ups? Lt. Green: Miss Evelyn cycled hundreds of kids out of the foster program and into permanent homes. In 30 years she only came across four lost causes. Four delinquents so far gone she couldn't find anyone to take them in. So she did. Trust me, Fowler, these kids are congressmen compared to what they would've been.
Happy Gilmore: Happy's grandmother is his defining moral point — her being in trouble is what sparks the whole plot off, and Happy rejects anything that will cause her harm in some way.
The title character in Harry Brown loses his wife and best friend in the first few minutes. With them gone, there's nothing left (except advanced age) to keep him from reverting to the ruthless torture-using Royal Marine he once was.
Shilo acts as one of these for her father, Nathan in Repo! The Genetic Opera. She replaced her mother, Marni, who was more of a Morality Pet. It's implied that without Shilo, Nathan would take on the Repo Man persona full time.
Extensively played with in The Sons Of Katie Elder when the four sons return to their mother's home for her funeral and the three elder brothers (all "bad men" in the sense that they were Gunslingers and so forth) decide that the youngest needs to go to college and be respectable so that their mother can be honored. Of course, the youngest son wants to be like his older brothers. They proceed to inform him that he is now required to be the honor bearer for the whole family. This involves a significant amount of brotherly violence to make sure he understands.
Star Wars basically had Emperor Palpatine psyching Anakin/Darth Vader to the point of killing his own Morality Chain, Padmé.
For that matter, Return of the Jedi has Luke becoming a Morality Chain for his dying father.
Actually, they serve as each others' Morality Chains. Darth keeps Luke from giving in to the Dark Side, and Luke brings Anakin back to the Light. Leia also serves this function for Luke somewhat in that when Vader threatens her Luke jumps back into battle.
And Palpatine notes in the Revenge of the Sith novel that Obi-Wan is Anakin's other morality chain, with Dooku's plan being to break the chain by killing Obi-Wan. Palpatine has other ideas, of course. He also managed to weaken the effect of both of Anakin's chains; at the start of the novel Anakin has some respect for his wife's autonomy and doesn't want to hurt her. Later, even before he strangles her, he sees her less as a person he loves and more as a possession expressing fear and pain, emotions that are beautiful to behold because he earned them.
Palpatine also sows distrust between Anakin and his morality chains, framing Obi Wan and Padme's recognition of Palpatine's growing dictatorship as treason and betrayal of Anakin personally. He also is very careful to isolate Anakin from both of them, going so far as to manipulate the Jedi into sending Obi Wan half a galaxy away when the crux of the plan is in play.
Kota and Juno is generally Galen's morality chain, with Juno being the only person who he really cared about and loved.
There are only two people Daniel Plainview has any kind of love for; his adopted son H.W. and his half-brother Henry. But even this love has limits. Henry is revealed to be an impostor who took Henry's identity after he died of tuberculosis to find work; Daniel kills him in cold blood. He also grows increasingly frustrated with his son's recent deafness, until finally, when an adult H.W. respectfully asks to dissolve their partnership to seek his fortune, Daniel lashes out at him, telling H.W. of his real parentage and subjecting him to verbal abuse. H.W. ultimately walks away in disgust, leaving Daniel all alone.
In The Godfather, after he figures out the treachery of Carlo, the brother-in-law, Michael waits to assassinate him until the death of Vito, because he doesn't want to hurt the old man by breaking up the family. After Vito dies of a heart attack, all bets are off.
Taken even further in The Godfather Part II, where Mike finds out that his actual brother, Fredo, has betrayed the family. He explicitly tells Fredo that he is safe for now, because Mike knows what killing him would do to their mama. After Mama dies, though...
The second half of the orignal Slayers novels (the ninth-through-fifteenth novels, which were never translated into English) introduced a new rival to Lina, Luke. He is an ex-assassin, and the only reason he never turned back is because of his partner Milina (and it' shown that he is clearly in love with her). With her he is more or less a Jerk Ass, but when Milina is killed later on, Luke becomes a crazed Omnicidal Maniac, allowing his piece of Shabranigdo, the Big Bad, to awaken out of his soul.
Arguably, Dexter's foster father, Harry, who at least managed to train Dexter to select his targets according to certain standards of morality and justice.
Turned on its head in the Discworld novel Witches Abroad. Granny Weatherwax is good because her sister turned evil, which by the Theory of Narrative Causality meant she had to be the 'good' one to balance things out. Granny never forgave her sister for that, since she maintains adamantly that she'd be an infinitely better villain than the sister. It should be noted that Granny was most pissed off because she never got to choose. She had to be the Good One because her sister was the bad one. That the sister thought she was the Good One was simply the icing on the cake.
Witches as a "community" keep an eye on each other for this very reason. Any group of witches larger than  has a tendency to fall apart from bickering, but there's still a silent understanding that some level of contact helps them avoid "going to the bad".
In the Prince Roger books by David Weber and John Ringo, Sergeant Nimashet Despreaux is told by a number of her comrades that she must survive because she is the Morality Chain for Prince Roger MacClintock, who will be the next emperor of mankind. She is also told that is why she has to marry him.
Zigzagged with Snape in Harry Potter: though his love for Lily was not enough to keep him from joining the Death Eaters, Voldemort threatening her life switched him to good, and her death (and his responsibility for it) was the main reason he dedicated his life to protecting Harry, even though the two loathed each other.
Downplayed with Aragog and Hagrid. Aragog's loyalty and devotion to Hagrid were strong enough to suppress Aragog's natural instinct as a dangerous magical creature to eat humans. This only went so far; Aragog didn't try to deny his family the opportunity to eat Harry and Ron when they visited him in the Forbidden Forest, though he would prevent them from trying the same with Hagrid. Of course, whether or not a sapient monster (whose instincts drive it to eat people) actually eating people could be considered "evil" is a tricky matter.
In The Demon's Lexicon by Sarah Rees-Brennan, the protagonist's brother Alan acts as a Morality Chain for him.
Invoked in the Warhammer 40000 novel Deus Sanguinius. When Rafen offers to challenge Arkio's claim to be Sanguinius reincarnated in combat, Stele is glad despite the unexpected turn Rafen's survival has caused, because he believes that Rafen is the last link Arkio has to his humanity and that Rafen's death will cause Arkio to be permanently lost to Chaos.
"One Lonely Night" begins with Private DetectiveMike Hammer angsting over being chewed out by a judge merely for blowing away a scumbag who likely would have been executed anyway. He spends much of the book bothered by his Honor's assertion that he's no better than the man he killed, and so is quite pleased when he's able to retrieve the MacGuffin without killing many more people. Then his Sexy Secretary Velda gets kidnapped and Mike realises he's been kidding himself. Of course he's an evil murdering bastard. That's what he was put on Earth for! He then proceeds to track down the Dirty Communists who kidnapped his secretary and Kill Them All.
In the novelization of Metropolis, Hel functioned as one of these for both Joh Fredersen and Rotwang. After she dies, Joh withdraws completely into work and becomes a cold-hearted executive, and Rotwang withdraws into his laboratory to spend all his time plotting revenge and building a robotclone of Hel. At the end, Joh Fredersen is redeemed by his son Freder, who becomes his new morality chain.
In the Vorkosigan Saga the sadistic Sociopathic Soldier Sergeant Bothari has two morality chains: his daughter Elena and the young Miles Vorkosigan. Lacking any moral sense, he uses duty to dictate how he treats them.
While the cold and ruthless Tywin Lannister of A Song of Ice and Fire was never really a nice guy (he wiped out two families of rebellious vassals as a young man), it's mentioned that he used to be somewhat more humane, and in particular, showed his most visible happiness and emotion around his beloved wife. When she died in childbirth, all of that humanity was gone for good.
Relatively benign example in Luke Skywalker and the Shadows of Mindor. Nick Rostu was mind-controlled into attacking Luke, who took a chance and saved Nick. Now Nick can return the favor by gunning down thirty-odd mind controlled people and saving Luke... but... he hesitates
because he had an overpowering intuition: if Luke Skywalker thought he might save thirty innocent lives by sacrificing his own, he wouldn't hesitate. Ten innocent lives.
"Or, hell, one not-so-innocent life," Nick muttered. "Like mine." He flipped the carbine's power setting to stun. "I hate Jedi."
Christine in "Beauty and the Opera, or the Phantom Beast," by Suzy Mc Kee Charnas. The Phantom's whole idea of morality consists of obeying Christine's decree that he must not murder people who annoy him.
Catherine Earnshaw is this to Heathcliff in Wuthering Heights. He was never a nice person (though this may be a case of Then Let Me Be Evil, considering that everyone ostracizes him and expects him to be evil from the start because he's an Ambiguously Brown rescued street urchin), but once she marries another man and then dies in childbirth, he develops into a near-demonic Villain Protagonist and takes elaborate revenge on everyone who has ever kept him from Catherine or otherwise screwed him over (and theirchildren).
In the Xena: Warrior Princess episode "Dirty Half Dozen," Xena says that Gabrielle is something of a morality chain and confirmed in seasons 5 & 6; "Gabrielle" throws away her staff, "Xena" and her minions commit genocide versus gods, centaurs, Amazons, Japanese etc.
On As the World Turns: Many characters believe that Luke is the reason that Reid is becoming nicer and more human... and they're right.
Connor and Cordelia in Angel have an aspect of this influencing him to a reconciliation with Angel, and giving him a much-needed connection to a non-dead and crazy human being. As Connor becomes increasingly dependent on her for guidance, she becomes increasingly Not Herself, and asks him to get her something very important.
Team Angel, as a whole, is Angel's Morality Chain. Notably, during Season Two, he abandons them, believing they are making him "weak", and getting in the way of his fight against evil. Without them, he begins to lose more and more of his humanity, until he sleeps with Darla in an attempt to lose his soul and become Angelus.
Dexter: Harry once again, though his family (Rita, the kids, his sister,) are like reinforcing links on the chain.
Doing nothing to stop the accusations of Ho Yay, Supernatural did this in "Mystery Spot". Dean is really gone this time and while Sam isn't exactly 'evil', he's a more unhinged, colder mix of John and Gordon. While it didn't last (although there was pretty crappy after-effects on Sam's mental state), it still showcased their Sibling Yin-Yang awesomely: If it weren't for Sam then Dean would commit suicide and if it weren't for Dean then Sam would lose his innate humanity.
In Season 1 and early Season 2, there are suggestions in Sam's moralizing that he's going to be the morality chain for Dean. As the troper above indicated, this gets turned around hard in later seasons.
Sam didn't get addicted to demon blood until Dean was no longer there to stop him. Sam has generally good intentions, but he went from a deontological moralizer in Season 1 and parts of Season 2 (won't kill humans, doesn't like moral ambiguity, wants Dean to kill him if he goes darkside) to a fervent consequentialist by the beginning of Season 4 ("It's totally cool to bang a demon chick as long as she's in a (mostly) dead body and also to guzzle down demon blood because it lets me save people.") Sam's motives don't fundamentally change, but failing to stop Dean's descent into Hell drives him all the way off the slippery slope on the sliding scale of means vs. ends.
At the end of Season 4, Sam brutalizes Dean and nearly chokes him to death before relenting, to which Dean responds with an extremely ill-advised ultimatum mirroring their father's when Sam left for college, and EVEN THEN, Sam's trust in and love for Dean very nearly prevent him from engaging in the final orgy of demon blood and violence that freed Satan from hell just as he's about to do it. It's only because Zachariah edited the reconciliatory phone message Dean left on Sam's voice mail that Sam finally comes to believe that Dean has abandoned him, and that he has to go through with his plan.
Likewise, in a Season 5 episode The End, Dean is sent to a future where he and Sam never reunite and Sam ends up becoming Lucifer's vessel. Even Dean has to admit that his future self is a callous dick and by the end of the episode, has come to the conclusion that they "keep each other human."
Alternate Future Sylar from Volume 3 of Heroes has his son Noah as his Morality Chain, one strong enough to subvert Do Not Call Me Gabriel. With Noah around, Sylar was able to curb his hunger and become a good man. But then it all goes horribly, horriblywrong when it turns out that Knox doesn't know the meaning of Infant Immortality.
Davis Bloome from Smallville transforms into killing machine Doomsday with enough agitation; however, Chloe Sullivan's physical presence is the only thing that could calm him enough to stop the transformations. When he finds out Chloe truly loves Jimmy Olsen instead of him and that she ran away with him just to protect Clark, though, Davis (the human) goes apeshit and murderous.
Ianto Jones from Torchwood fills this role for Captain Jack Harkness. Word Of God has it that if he hadn't died, Jack would not have been able to kill his grandson later on.
Lady Adira and Vir Cotto to Londo Mollari in Babylon 5.It doesn't end well.
Well, Not for him. Or for Adira. But by all accounts, Emperor Cotto leaves the Centauri Republic far better than he found it, or indeed better than Cartagia had found it years earlier—and he wouldn't have become Emperor were it not for Londo's Thanatos Gambit. Babylon 5: reminding you that you have to take the good with the bad since 1994.
Lilly Truscott of Hannah Montana is quite often the only thing reigning in Miley's diva-ness.
The various companions on Doctor Who sometimes fill this role, though how much this is needed varies across his various incarnations.
Although toyed with quite a bit over the years, this is finally fully explored in the revived series, when the Tenth Doctor rejects the notion of companionship altogether after losing Donna, and ends up arrogantly deciding that the rules of time no longer apply to him. It's more of a Screw the Rules, I Make Them. "The Laws of Time are mine, and they will OBEY me." He goes so far as to change a vital point in history, albeit with the aim of saving a life. The woman whose life he saved, realizing the potential consequences, immediately kills herself. This causes a contrite Doctor to realize how crucial his companions really are.
Donna even says outright in "The Runaway Bride" that the Doctor needs to find someone who can serve this purpose for him.
This resurfaces with the Eleventh Doctor, as he tries shifting to a part-time companion model, dropping in on them from time to time. Turns out, well-intentioned as the idea might be, going without companions for long periods still has an erosive effect on his morality.
Subverted with The Master: the Doctor pretty much tries to be a Morality Chain for him. Without much success, so far.
One can argue that it's played straight with the Master — the fact that he doesn't need companions is a sign of his sociopathy. He might have sardonically referred to Mrs Saxon as "my faithful companion", but she's just another Weak Willed subordinate he dominates through mind control as per his Seventies incarnation.
In Firefly, Mal is Jayne's Morality Chain, though it's not that Mal makes Jayne good so much as Mal is willing to toss him out the airlock if he betrays any of the crew.
One could argue that the entire crew (sans Simon and River) is something of a more classic morality chain to him, probably due to them being a strong sense of family, he hasn't had as a mercenary (although there's hints he had one as a child). This is shown most in the same scene where he's more concerned about how the crew thinks of him after his death than him actually being dead.
On Charmed, many characters have explicitly stated that Phoebe plays this role in making sure that Cole remains good. In fact, Cole's well-known tendency for being a Heel Face Revolving Door is pretty much a function of his relationship with Phoebe—that is, Phoebe and Cole are constantly breaking up and reestablishing their relationship, and every time this happens, it has a major effect on Cole's status as evil or good. Although Phoebe is very much aware of her power to be Cole's Morality Chain, she usually seems to be merely yanking his chain based on her own emotions, rather than consistently using her influence to make sure Cole becomes and remains a redeemed demon.
Used in some episodes of Criminal Minds. In fact, one of the killers outright stated that as long as his morality chain was with him, he would do no harm. Another episode has a close examination of the strange, symbiotic relationship between the killer and his chain that ends in a truly heartbreaking fashion.
True Blood: Russell Edgington, Big Bad of Season 3, had one in his lover Talbot. While he was always clearly the bad guy, he spent the first two thirds of the season being a Faux Affably EvilMagnificent Bastard who spent most of his time monologing, trying to seduce the main characters to his side. However, once Talbot was murdered, shit got real.
Eric: "Russell Edgington was maybe the oldest and strongest vampire on the planet. Now he is also the craziest."
In Being Human, Mitchell describes this trope - when asked by another vampire how he lives with his Horror Hunger without succumbing, he says: "you surround yourself with good people, that's what you do. You find someone better than you. Cause then when you fail, you have to deal with their disappointment."
In the fourth season, Hal goes out to kill a man, like, ten minutes after losing his family. He stops because Annie told him how much they would be disappointed.
House explicitly asks Martha Masters to become one for him. Without someone acting as an ethical compass, he stands to lose Cuddy.
An episode of Fringe involved a man and his counterpart from the other dimension both of whom struggled with an inner "darkness" as a child (resulting in things like mutilating animals), one ended up a memory stealing serial killer, the other was a well adjusted criminal psychologist because he encountered and developed a relationship with a loving mother figure in his youth. Eventually the two cross paths and the killer steals his memories of the mother figure then kills himself out of remorse when those memories take root and make capable of understanding how monstrous his actions have been. The FBI is worried that the surviving psychologist (now suffering from partial amnesia regarding the woman who saved him) will become a killer himself, but it's implied that he still retains things she taught him and her influence persists even without being able to remember her specifically.
Ted is something of this to Barney in How I Met Your Mother, and Barney has been shown to forgo his more despicable actions if he's afraid of pushing Ted too far. Notably, when he has a one-night stand with a recently-dumped Robin, Ted's ex-girlfriend (and very close friend), Ted does temporarily break off their friendship, which sends Barney into a spiral of depression, desperation, and revenge plots. Ironically, this may have been the first time in Barney's life when he was actually being kind and comforting instead of just using a girl for sex, but given Barney's history, it's hard to blame Ted for not believing him.
Leverage is intersting in that there are sort of two characters fitting this role. Nate is this to the rest of the team, causing them to do good. On the other hand, he is willing to often go too far in cons unless Sophie stops him. Sophie's importance is especially seen during season 2 when she temporarily leaves and he begins to go too far.
In Star Trek: Deep Space Nine, the Changeling Founders (Odo's people) lead the tyrannical Dominion and wage war against the Alpha Quadrant powers. Laas argues that Kira is the only reason why Odo hasn't left Deep Space Nine and joined the Dominion.
Odo: I won't have anything to do with the Founders and their war.
Laas: Odo, we linked. I know the truth. You stayed here because of Kira. If it weren't for her, you would be with our people. War or no war, you would be a Founder!
Also has another example of one that ends very badly with Gul Dukat's daughter, Ziyal. While she's around, Dukat begins to turn over a new leaf but after his second banana Damar accidentally shoots her dead, Dukat goes completely off the deep end and tries to sacrifice the entire universe to a bunch of alien demons.
In Once Upon a Time, Belle is this for Gold/Rumplestiltskin. He tries to be good for her, even though he is still pretty bad. Without her there is nothing holding him back from being downright evil and killing whoever he wants. Belle can also be seen as a Morality Pet, since she is the living embodiment of Pet the Dog. She is the only one that Rumple is nice and sweet to, his only weakness. If anyone messes with her they are toast. He even says in season 2 episode 12 that if any harm comes to Belle while he is gone, he will kill everyone! Due to her, we see that Rumple does have a heart after all. His lost son will also probably serve as a morality chain for him once he finds him.
The Breaker: Shi Woon Lee and Shiho are this to Chun Woo. Shiho isn't as much of one as Shi Woon but her apparent death provokes quite a reaction in Chun Woo.
Role Playing Games
In Survival of the Fittest, Elizabeth Priestly is this to twin brother Lenny. When she's not around him, he acts even more of a complete bastard to get her back/find her.
And now that she's permanently out of the picture, we can probably expect even nastier things to happen to anybody Lenny meets...especially Gabe McCallum.
In Rifts, Baarrtk Krror is only prevented from giving in entirely to hate by his dear friend, Malik Savant.
The Vampire The Requiem supplement Danse Macabre introduces the concept of "anchors" as a replacement for the Humanity system. The way it works, essentially, is that you have a set of Morality Chains that prevent you from degenerating and giving into The Beast. You lose an anchor when they become too exposed to the rest of vampiric society or you damage the relationship too thoroughly. Did we mention that the anchor system is meant to be used in conjunction with the Atrocity system and your anchors are the easiest way to safely vent Atrocity dice?
Inverted in The Legend Of Zelda Twilight Princess. Midna spends the first third or more of the game thinking only of her own world and recovering the things that she needs. The selflessness she discovers in the personalities of Link and Princess Zelda, however, become a Morality Chain for her which develops over time, to the point where she cares more about them than she does about herself.
In La Pucelle, the Dark Prince became such when his love interest is killed. Even after Amnesiac Dissonance, he reverts to such when someone who looks like said love interest is in the same situation that killed his original love, and reverted completely when he witnessed the original event again. In Ragnarok, Croix (The Dark Prince) himself is a Morality Chain for Overlord Priere.
Bastila and Revan in Knights of the Old Republic serve as each others' Morality Chain, as their telepathic bond allows them to subtly influence each other towards the Dark or the Light. The ending for Lightside!Revan is actually something of a Morality Chain Tug-Of-War, with Bastila trying to use the connection to convince Revan to fall with her and Revan using it to support his "I Know You're in There Somewhere" Fight speech.
The mother of the villains Isair and Madae in Icewind Dale 2, although her death happened before the beginning of the story.
Raspberyl to Mao in Disgaea 3. When she dies in the worst ending, he completely loses it.
Flonne could also be seen as becoming one to Laharl gradually throughout Disgaea. In the Normal Ending, when he learns that he's actually responsible for killing her, he sacrifices himself to bring her back to life, effectively inverting this trope.
Played straight in Disgaea 2, though reversing the roles. Rozalin warms up to Adell's strong sense of justice, his family, and his determination to bring her back safely to her father. In the worst ending, unlocked by having too many ally kills, after finding out Rozalin is actually an extremely powerful demon overlord who sealed herself in an attempt to escape a life of violence, she judges Adell to be a creature of sin and it is presumed she destroys everyone. This, of course, is avoided in the good ending where the Adell who has lived up to his ideals is able to bring her back to her senses with the Powerof Love.
And, in the worst ending, Adell (in addition to failing his Morality Chain duties) is forced to kill Rozalin when her Super-Powered Evil Side is unleashed. Unfortunately, her now-bodiless evil side simply possesses Adell and the screen goes black. Then we get to listen as Adell kills his little brother and sister while they beg and plead for him to stop.
If the Japanese audio is any indication, he then proceeds to eat them.
One of the odder cases, in that Maria is not so much the thing keeping Shadow on the side of good as she for a long time the only thing motivating him to do anything at all - good or evil. His belief that she wished him to avenge her motivates him to attempt to kill everyone on the planet - including himself - by bringing a Space Station he lived on out of orbit. His belief that she wanted peace for all motivates him to save the planet from the station, but entirely for her sake without pretence that it's his own goal or even morality that he's following. After which he may even have attempted to kill himself by falling into the atmosphere. Maria isn't so much the morality chain keeping him on the side of good as she is the morality chain giving him any sort of morality at all. Without her input he could be considered almost amoral and certainly apathetic.
In Planescape: Torment you meet a devil called Fhjull Forked-Tongue. He made an angel called Trias enter into a Deal with the Devil with him, hoping to corrupt Trias to evil. Unfortunately, Trias was better at contract manipulation, and Fjull got stuck with the short straw: By the contract, he bound to be good for as long as Trias is alive; breaking the contract would mean his death. Trias, on his end, does not have to fulfill any part of the contract for as long as Fjull is still in the process of fulfilling his own stipulations (in other words, Trias gets off scot-free until Fhjull willingly becomes Lawful Good, at which point Fhjull wouldn't want Trias to fulfil his part of the bargain). Although forced to be good, Fjull is still allowed to be bitter about it, which he is. Oh so very much.
In World of Warcraft: Cataclysm, the night elf Leyara is a key member of Fandral Staghelm's Druids of the Flame, who answer to the malevolent Old God-aligned Ragnaros the Firelord. She goes down this path after her daughter Astaria is slaughtered by Horde raiders invading their village, and is aggrieved by Malfurion's belief that interfaction warfare is too much of a distraction from saving the world being torched by Ragnaros. The fact that Leyara is Fandral Staghelm's daughter-in-law also seems to have something to do with her downward spiral, given that Fandral had previously kept Malfurion trapped in the Emerald Nightmare and has become Ragnaros' new right-hand man.
In Dead Rising 2: Off The Record, losing Katey in this continuity turns Chuck into a psychotic alcoholic and one of the Psychopath bosses for Frank to deal with.
In The Elder Scrolls games, Clavicus Vile the Daedric Prince of Wishes and Deals is fond of making deals that often screw over foolish mortals. His hound Barbas is the manifestation of his conscience who tries his best to keep his master from ruining too many lives.
In Half Minute Hero, whenever something bad happens to Millennia, Beautiful Evil Lord goes absolutely berserk.
Grand Cleric Elthina of Dragon Age II is a morality chain to Sebastian. Once she's killed, Sebastian swears to raze the whole city in vengeance if Hawke chooses not to kill Anders, her murderer.
A Hawke who takes the romance or friendship path with Anders is this to him and Justice until thethird act. Likewise, Hawke can garner enough relationship points to stop Fenris from killing his long-lost sister after she sells him out to his former master but can't prevent Merrill from working on the Eluvian that will eventually take out her mentor and possibly her entire clan or Isabela from running off with the Qunari relic—atfirst.
A Paragon Commander Shepard in Mass Effect plays this role to nearly any character with an ounce of anti-heroic traits. Notable examples are Garrus, Liara, Jack, Miranda, and Aria T'Loak.
In StarCraft II: Heart of the Swarm, Jim Raynor is this (to some extent at least) for Sarah Kerrigan. When Kerrigan thinks Raynor's dead, she goes back to the Swarm so she can exact vengeance on Mengsk, and she becomes rather ruthless in pursuit of this goal. When she later finds out that Jim is actually alive (shortly after getting herself re-infested as a primal zerg), her ruthlessness dials back somewhat, to the point that when she finally does arrive at Korhal to exact her vengeance, she tries to avoid civilian casualties (at the request of Valerian Mengsk). Kerrigan never completely turns evil (even at her worst, she seems to have more of a moral code than she ever did as the Queen of Blades), but she's definitely a worse person when she thinks Jim's dead than when she knows he's alive.
Kerrigan herself is this to the rest of the Swarm now that she's regained her humanity. Zerg on their own view every other living thing as a threat to be destroyed or assimilated so Kerrigan steers them away from innocents and refuses to let them attack allies once they're finished being useful.
Dan's mother Destania from Dan and Mab's Furry Adventures was a powerful, manipulative, cunning and utterly ruthless Succubus with little to no regard for Beings (the "Normals" of the setting). Then she fell in love with Dan's adventurer father and gave birth to Dan and grew to love life as a loving mother to Dan and his half-sister Alexi; so much so that she renounced her previous life as Destania. Then Dan's father was kidnapped by dragons. While scheming to get him back, and without the presence of her children, Destania is slowly but surely reverting to her past villainous self.
Well, maybe. She has left Lost Lake Inn to protect her children, the plot she's involved in, if successful, will eliminate not only the threat to her family, but a major re-occurring threat to her kind. And if the plot seems genocidal, that seems to have been the nature of the conflict already.
Roy to Belkar in The Order of the Stick. Without Roy (and by extension, the rest of the party) to play the role of commanding officer and coax him into coming along on their adventurers under the occasional threat of force, Belkar would be dropping the 'heroic' part of Heroic Comedic Sociopathvery quickly. This is given a Lampshade Hanging when Roy goes to the afterlife and a character reveals that Belkar's growth on the 'Evil-O-Meter' began to reverse after he signed on with the Order. Note that Roy isn't making Belkar behave any less evil — he's merely got him pointed in the direction of people who deserve it slightly more.
Not only was it reversing a little without Roy, Roy had actually successfully reined it in to a relatively low level, whereas without Roy's intervention he might have ended up a villain nasty enough to rival Xykon (for perspective, the scale upon which Belkar was being measured, was being measured in kiloNazis)
His evilness level would have quickly gone to 8 within a year without Roy's guidance (that surpasses the hypothetical offspring of Sauron and Cruella de Ville by 3 KiloNazis). That would have been pretty freaking evil.
Belkar's cat, Mr. Scruffy, seems to be becoming this as well. Belkar's almost definitely faking it, though.
It doesn't always work, though - Vriska's moirail Kanaya does a terrible job as her Morality Chain (partly due to a romantic conflict of interest), and Vriska freely swings between Jerk Ass and Faux Affably Evil. Plus there's the fact that moirails can actively "break up" their moirallegiance with their partner in much the same fashion as a typical romantic relationship...
It might not be quite "morality", per se, but the Draconian Dignitary's influence and nagging is pretty much the only thing keeping Jack Noir from killing everyone and obliterating everything, and instead keeping his murder and Earth-Shattering Kabooms reasonably limited to ensure that Derse still has some people to oppress and tax.
At this point, Karkat is probably this for Gamzee, as the creation of their moirallegiance was the only thing stopping his murder spree, but at this point no-one knows whether Gamzee would revert back without his influence.
After Mike's death in College Roomies from Hell!!!, Marsha comments on her insanity, saying he was the rope on her catapult. Now he's gone, and she can soar...
In El Goonish Shive, all known Alternate Universe variants of Tedd got a good friend Elliot and at least once the female counterpart Ellen is his girlfriend; the only exclusion is Lord Tedd's world — and instead of Grace there's a big ill-mannered Blood Knight.
Chuckie is this to Tommy, Phil, and Lil on occasion in Rugrats, particularly when they are being led astray by Angelica. In "Rebel Without a Teddy Bear" Chuckie's vocal intervention is the only thing that keeps Tommy from becoming as much of a mean-spirited hellion as Angelica.
Other episodes like "The Gold Rush" and "Chuckie's Wonderful Life" strongly hint that Chuckie is this to the other babies as well.
Further, the drastic expansion of the League was partly this; when Green Arrow just about leads a coup against the founding-seven because a large number of the members felt they were starting to abuse their powers (not to mention that they just revealed the Watchtower doubled as a Kill Sat), they're thanked and reminded that half the reason that the more Badass Normal heroes and the ones more in-tune with the life on the street are there to keep the borderline-Physical God heroes down to earth, on top of being amazing (and often underrated) heroes in their own right.
In Teen Titans, Starfire is thrown into a temporal vortex in the episode "How Long Is Forever?". She lands in a world where she's been lost for several years, and it turns out that the Titans were so dispirited and sad after her disappearance, that they have been disbanded and haven't seen each other for years. BB is a freak show in a circus, Raven went the maddened into misanthropy way, Cyborg has spent years alone in the remains of the Titans Tower, and only Robin (now Nightwing) remains as a crimefighter.