Villains can become popular enough that the fandom becomes sympathetic to them. However, meshing them into the main cast can be dangerous and disappointing if Good Is Dumb. This can also be unrealistic since even those downgraded to protagonists tend to be unsure of anyone trying to help them. Some heroes wouldn't exactly be quick to accept them, either.
One way to handle this is to present another character to befriend them. Ideally, this should be someone who could pose no threat or even be demanding, and depending on how the plot goes, the recovering villain becomes fond of this other character and may draw stability from their presence. The Morality Pet may even be somewhat pathetic (and will be told so), but has often gone through some anguish to which the recovering villain can relate secretly, or reminds the villain of someone he lost long ago. They're usually upbeat, though they might go a little overboard sometimes. If handled poorly, they might become a Satellite Character for the former villain.
This also makes for a convenient excuse to help the heroes if needed; an ex-villain can simply claim to have his own 'selfish reasons' and save face. Anyone messing with the Morality Pet can expect a full-on Mama Bear or Papa Wolf response from the ex-villain or antihero in question.
They are a living and recurring incarnation of Pet the Dog, i.e., something to placate the audience regarding the hero's tolerance of the amoral character, while focusing on the true villains. (Unless they Kick the Morality Pet, that is, which leads their character arc in a different direction.)
This isn't just for villains, either. Despite sidekicks being less popular in comic books than they were originally, some writers pair them with Anti Heroes in an effort to humanize them.
Not to be confused with a magic-user's familiar, the creature type of which often reveals the kind of person who owns it, though if the magic-user is fond of the familiar and treats it well, it could very well be a Morality Pet.
If the Morality Pet dies or disappears and this causes the villain/anti-hero to revert back to his or her old ways, the Morality Pet was acting more like a Morality Chain.
Parent trope of Little Brother Is Watching.
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Anime and Manga
In Get Backers Makubex first appears as a Big Bad and manages to keep his cool down most of the time. When Sakura Kakei, a woman close to him, almost dies in combat Makubex hates himself for what happened.
Reimei No Arcana: Nakaba is one for both Loki and Caesar. Vernas' younger sister Lemilia is one for Vernas.
Nabari No Ou: If it weren't for Gau, Raikou would basically be a villain.
Yachiru Kusajishi from Bleach is the adorable child vice captain of Kenpachi Zaraki, one of the most violent and physically monstrous of the 13 captains that rule Soul Society. 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.
Note that Yachiru enthusiastically encourages "Ken-chan" to go wild when he gets a chance to fight. And is STILL a positive influence on him.
Rangiku towards Gin. Especially evident when it's revealed the only reason why he joined Aizen was to get revenge because Aizen's underlings and he injured Rangiku in the past.
Lilinette seems to serve as this for Stark. When Aaroniero was killed, Stark went right on sleeping until Lilinette woke him up and asked him what he was going to do about it.
In InuYasha, Rin's humanizing (so to speak) effect on Sesshoumaru. Her introduction was the start of his shift from Villain to Nominal Hero and finally to full on Aloof Ally.
Kohaku became like this for Kagura. Especially notable because later, she ends up dying for him.
Mokuba Kaiba from Yu-Gi-Oh! sometimes takes this role, making Seto Kaiba somewhat approachable. This, however, is averted in the manga, where, in the arc where Seto is actually a villain, not only does Mokuba epically fail to rein in his brother's sadistic tendencies, he is actually a willing accomplice to Seto's Evil Plan. After Seto's Mind Crush, Mokuba plays this straight.
In the first anime Bradley's pet was his son, Selim. Until he appeared he gave off an ambigious air after how easily he killed Marta and his reveal as being an homunculus. Selim showed off he had a nice side too. Or really how great he is with facades. He ends up killing his Morality Pet before he himself dies
Wormmon to his partner Ken Ichijouji from Digimon Adventure 02, after Ken's repentance. (Unlike the standard depiction of this trople, when Ken was evil, Wormmon was basically the dog, as in "kick the.")
Not to mention Gatomon with a Morality Pet Wizardmon in the original.
And Gatomon's eventual partner replaced that wizard when the poor wizard kicked the bucket. Kari being a Morality Pet to something normally considered a pet?
Kari reached morality pet status before that. The assigment was to kill the kid, but when the time came, she just couldn't.
In Magical Girl Lyrical Nanoha, Hayate acts as a morality pet to the Wolkenritter, especially Vita, who was rather hostile to everyone before Hayate appeared, and Signum, who was extremely serious and emotionless about her "job".
Speaking of C.C., one might say that Lelouch becomes a Morality Pet to her, in that she begins to feel human emotions once again while in his presence, even against her own admission sometimes.
Downplayed with Ralph in Soukou No Strain is kind to the living Emily and protects her from the rest of the Deague, but when he throws a coup on the ship and kills everyone but Emily and Medlock, it's clear that even that won't save him. He's insane, and if he's allowed to live, he'll turn on Emily just like his real little sister, Sara. He does find a glimmer of sanity near the end, though.
Dragon Ball's straight application of this trope would have to be Piccolo and Gohan. While Goku's influence means Piccolo is no longer evil, essentially adopting and training Gohan made him proactively heroic. He is then promptly hit with Redemption Equals Death, though not permanently.
Kazuma of Kaze no Stigma acts distinctly uninterested in helping people or saving the day unless he can extort a profit... but he cares a great deal about his little brother, Ren, and will not hesitate to save him...and he'll still charge you for it in order to keep up his image.
Alyssa Searrs is Miyu's Morality Pet in Mai-HiME. When Alyssa is introduced in episode 9, Miyu (who is the daughter/creation of Father Joseph, a priest at the school's chapel) is actually seen crying, genuinely moved by her young protectorate's singing abilities.
Of course, this is then subverted when you find out what Alyssa is a 'fake HiME' created by Searrs, who is a living remote control for an orbiting Kill Sat she is not afraid to use to hold the school hostage with. Oh, and she has a private army.
This may have something to do with threatening Garcia being her Berserk Button as well, however.
Casca and Puck act as this for Guts from Berserk following the events of the Eclipse that shattered Casca's mind. Guts's concern for Casca's welfare in particular, and his need to protect her from the same demons that constantly hunt him due to her having the same Brand of Sacrifice that Guts has is about the only thing that keeps him from becoming completely consumed by his hatred for Griffith and the Godhand.
Collectively speaking, all of Guts' current True Companions act as his voice of reason.
Another Apostle, the Count, had a daughter named Theresia, the only thing that remained of the family he had before he became an Apostle. While he still was very evil, his reactions to her show that he was quite more than a mere monster. When the Count lost against Guts, summoned the Godhand with his Behelit, and was told by Femto to sacrifice Theresia so he could be reborn or be condemned to Hell, he chose Hell over handing poor Theresia over.
In a tragic aversion Casca and the band of the hawk were almost this for Griffith/Femto. First, when Griffith is permanently crippled and right at the breaking point to use the behelit he stops and considers the peaceful life he could have with Caska and calms down, her being the one thing that could make him give up on his ambition, but then when he overhears Caska planning to run away with Gutts it pushes him right off the edge. And then after becoming Femto when he returns to the physical world, he visits the memorial for the band of hawks and sees Caska just to prove to himself that he doesn't have a morality pet.
In Spider Riders, Princess Sparkle plays this role to Grasshop, eventually leading him to switch sides. Also, to an extent, Aqune to Buguese.
In Mahou Sensei Negima! (the manga, at least), Chachazero serves as a reverse morality pet to Evangeline. She's generally harmless when encountered, but being basically a serial killer in the body of a puppet, and designed as such by Eva, her comments and reminiscing about the bad old days demonstrate how much her creator's softened since. At times, Chachamaru verges on the straight version of this trope, but tends to stay in Battle Butler territory.
Fate's group of female minions serve as his Morality Pets. They serve him so loyally because they are all war orphans he rescued and took in, and they chose to aid him in his mission in return. He actually took in around fifty such orphans; those that didn't want to join Cosmo Entelecheia were were given a decent education at Fate's expense.
While he isn't really evil, Kaze's concern for Takeshi in Gantz definitely reveals kindness beyond his Arrogant Kung-Fu Guy image.
More notable is the effect Tae Kojima had on main character Kei Kurono, practically single-handedly turning him from a Jerk Ass to an outright hero.
Goldie Musou, arguably the worst Big Bad in the entirety of Gunsmith Cats, had a literal morality pet. In a flashback to her past, we see her grandfather give her a dog. After her parents are assassinated by a rival mob and she takes over in their stead, the dog growls at her and runs away. It smelled the chemicals on her hands from her mixing the prototype version of the drug Kerasene that she'd later use to make her criminal empire a dominant power. She even gets An Aesop from her grandfather's echoed words. "The truest test is to let something go. If it doesn't come back on its own, it wasn't yours to begin with."
Kurita could be considered Hiruma's (enormous) morality pet in Eyeshield 21. Sure, Hiruma inflicts his over-the-top violence on Kurita just as much as anyone else, but the choices he made because of Kurita really show the "heroic" aspect of his sociopathy. Interestingly, Hiruma's actual pet, Cerberus, is, um. Not a Morality Pet.
In To Aru Majutsu no IndexLast Order acts this way for Accelerator who up to this point seemed like a mass murdering psychopath. Fairly credible job deconstructing his motives too. Though he's still a huge asshole if he doesn't like you. Then there was that one time he rescued a woman through the most violent means he could think of (slaughtering her captors to the last man).
Askeladd has a rather unusual one - the country of Wales, hismother's homeland that he would apparently rather die than see come to harm. He actually killed King Sweyn in front of his vassals to prevent him from invading.
Gunslinger Girl: Given that Guiseppe treats his cyborg assassin Henrietta far better than any of the other handlers, it's easy to forget that he too is a ruthless anti-terrorist operative, seeking revenge for his dead little sister Enrica. Lampshaded in a scene where a female politician crippled by a terrorist bomb comments on how cold and frightening Guiseppe seems. Henrietta, naturally, protests that he's "the kindest man in all of Italy!" The politician wisely lets the matter drop.
A strange inversion happens in Ranma ½. Akane adopts an adorable black piglet as her pet, which unbeknownst to her, is Ranma's exceedingly violent and hostile rival Ryoga in cursed animal form. As a result of her affectionate treatment, and because he's incredibly lonely due to his Walking the Earth lifestyle, being the pet causes Ryoga to fall hopelessly in love with her. This greatly softens his character and partially contributes to his eventual Heel-Face Turn (as he'll nearly always prioritize Akane over his grudge against Ranma, which allows for numerous Enemy Mine team-ups). Of course, there's the questionable morality of disguising himself as her pet in the first place, but hey...
From Shaman King: Hao's androgynous companion, Opacho, who uses toddler-speak and turns into a sheep. D'aww.
From Slayers: Not quite to the same extent, but Zelgadis acts far less like a Jerk Ass and more like a casual, if not aloof young man when Amelia's around. Compared to the other members of the group, he has an easier time softening up around her. Even though there is far less Ship Tease from them in the novels, he still gravitates toward her there.
A straighter example is Luke's bounty hunting partner Millina. She convinced him to end his assassination job, and he winds up falling for her to the point of gushing over everything she says since then.
Zwei, the cold-blooded assassin in Phantom of Inferno, adopts Cal, the kid sister of a dead hooker, in an attempt to protect her from his superiors. With a cute little kid tagging around behind him, he stops acting like the borderline villain he'd become and starts to ease back over into more standard Anti-Hero territory.
Cold, sociopathic gun-for-hire Katana from Gad Guard has the adorable little blonde girl Sayuri. Though he finds her annoying at first, after following him around for a while insisting she's his "fwend" and getting into danger a couple of times, she makes him see the error of his ways.
Double subverted in Yu-Gi-Oh! GX. Amon's little brother acted as one for him, but Yubel convinced him ditch his loyalty to the Garam group in order to fulfill his will and become king. After that, he sacrifices the woman who loved him and he loved back for power by unleashing Exodia. The double subversion comes in when he states that he will become king to make a perfect world so that no one suffers what he did nor can act on their deep, dark desires. This renders Yubel unable to feed off him, as he's not exactly 'evil.'
From Star Wars Manga, Darth Vader had a secret apprentice to soften him just a shade. No, not Starkiller. A Jedi child named Tao. Vader killed Tao's family, razed his planet from orbit, and spared Tao himself after being attacked by the child and sensing a lot of raw Force potential. Then he tried training the child, but since Defeat Means Friendship Tao refused to tap into his rage and hatred, endured Vader's frustration at this, and was eventually told that because of his weakness he was free to leave. ...Not something Sith say often.
And then the Emperor found out and had Vader mortally wound Tao, and for whatever reason Tao still didn't hate Vader, and was in fact glad that Vader was taking him to die on his homeworld. The homeworld that Vader razed. And when there, Tao saved him from a planet-hopping vengeful Tusken Raider, then told Vader that there was still good in him, before dying and making the Dark Lord feel remorse. This manga is quasicanon at best.
In Noein, cute little Miho serves as Jerk Ass villain Atori's Morality Pet. He was initially introduced as a Psycho for Hire villain but eventually gets bumped on the head and loses his memory as well as his Jerk Ass nature. He grew a fondness for her during this time, but after his memory (and true nature) returns and he starts being a jerk ass villain again, Miho is still able to bring back the dormant good and kindness in Atori, which eventually leads to his Heroic Sacrifice.
Holland is a major Jerk Ass toward Renton for a good part of the series, but he has a major soft spot for Eureka, practically to the point of a Big Brother Instinct. He has good reason to do so: she's part of the reason he came to his senses and abandoned the military death squads he served under.
The little kids seem to be this to Eureka. Flashbacks show her as having been cold, efficient, and mechanical, essentially a killing machine who only knew how to follow orders, when she was a part of the military. Then one day, she found three small kids that somehow survived the violent massacre that she just committed. Something about seeing them like that, vulnerable and utterly terrified, kick started something inside her, made her more human until she became the gentle, motherly girl she is when the series begins.
Inverted twice in Death Note, which has what could be called an 'immorality pet'. At first, Light (and the reader) believe that Ryuk, whose spare Death Note leads Light to become a Well-Intentioned ExtremistMagnificent Bastard, is his immorality pet. However, it's actually Light who is Ryuk's immorality pet, providing him with some interesting sick entertainment, and when Light no longer becomes interesting, Ryuk kills him.
Kind of played straight with Light's affection for his little sister Sayu and Mello's reaction to Matt's death, though.
That may have been true for pre-timeskip Light, but post timeskip, there's not much evidence that Light feels any kind of affection for his family, or anyone for that matter. To prevent his plans from being ruined, he attempts to kill her after she is abducted, but doesn't only because it wouldn't make sense for Kira to do that. He also shows no reaction at all to her near catatonic state following the abduction, meaning that Light likely sees Sayu as nothing more that another person he could potentially use to further his own goals, much like all the people he interacts with.
Bando from Elfen Lied has almost no redeeming features, being a JerkassBlood Knight who kills either for fun or revenge and beats up people for any slight provocation. But when he ends up basically homeless waiting for an opportunity to kill the person who almost killed and cripled him, he kind of becomes friendly towards Mayu, who has fled from her sexually abusing step-father and depressed mother. When she and her friend are assaulted by a man several times worse than him, he does get his opportunity to be a Big Damn Hero.
Chief Kurama has Nana, one of the mutants experimented on he has grown attached to and he tries to help as much as he can. While still taking part in the cruel experiments with the mutants. She is kind of a substitute for his own mutant daughter, who is so dangerous he tried to kill her after birth, which resulted in his wifes death when she tried to stop him, and their daughter growing up in a sealed vault fed by tubes.
Also Lucy has Kouta, but that was actually the reason the whole mess got started...
Keith Anyan in Toward the Terra has two, Sam Houston and Jonah Matsuka. Sam's interaction with Keith, particularly as the series progresses, always serves as an outlet for Keith's otherwise mostly repressed humanity; at one point Suena observes to herself that Sam is the only person Keith opens his heart to. Matsuka's relationship with Keith is a bit more tangled, but also ultimately humanizing.
Black Butler: Sebastian has his cat friends (and possibly Ciel himself depending on character interpretation—whether he genuinely cares about Ciel or if he wants nothing more than to eat his soul), Ciel has his household staff and Elizabeth.
Subversion: Lua Klein from Baccano! is a quiet, gentle Woman in White who seems like a perfect foil for her extremely enthusiasticPsycho for Hire squeeze Ladd Russo. However, she's really a suicidal masochist who seems to privately thrill at the idea of being murdered by Ladd, and Ladd's calm, sincere promises to kill her someday only emphasize his craziness.
However, it's played more straight when Ladd's arguably only redeeming act in the entire show is to throw himself off a moving train to save Lua, sacrificing his arm in the process.
Yuusei, the protagonist from Luck Stealer, is an assassin who will only kill criminals. This is because he wants to be a good person for the sake of his daughter.
Then there's Mukuro whose Heel-Face Turn is coupled with the introduction of Chrome, his Spirit Medium/Vessel, who he is shown to be rather fond of to the point that she is arguably his Berserk Button.
Shinra ends up fufilling this roll for Celty, his father Shingen and surprisingly Izaya. It's to be noted that Izaya leaves Shinra out of his plans more often then not, and DOES try to help him during the Valentine's Day special in the novel. Naturally, this might have to do with the two knowing each other since middle school and Izaya did something to him that makes him shut up immediately whenever Shinra brings it up. Turns out that Shinra protected him during middle school from someone that tried to kill Izaya with a knife by taking the wound for him; maturally, this is probably the only kind thing that has happened in Izaya's life.
Toriko is more neutral than anti-heroic at the start, but hanging out with Komatsu turns him into a proper hero. Komatsu ends up as the morality pet for other characters as well, including Sociopathic Hero Zebra and the creepy as hell Livebearer.
Krista is this to Ymir in Attack on Titan, tempering her worst behaviors and providing her with motivation to be heroic. Eren and Armin have shades for this for Annie, providing her most humanizing character development. Reiner suffers a particularly horrible example of Becoming the Mask, thanks to the others being this for him. In particular, he's shown to be protective of Armin, Connie, and Krista.
Alfred, Robin and the rest of the "Bat Family" function as Batman's ties to family and sanity. It's worth pointing that their ideas of sanity and/or morality doesn't necessarily fit with the rest of the world's, the Batfamily being what it is.
Wolverine has had a number of unofficial "sidekicks" used to balance out his ferocity and vaguely thuggish appearance. The tendency of them being young girls in most media — Jubilee, Kitty Pryde, a younger version of Rogue — is almost a cliche, possibly lampshaded in his humorous short alliance with Power Pack's five-year-old Katie Power. And when Molly Hayes visited X-Men Headquarters, guess who ended up babysitting her?
Only it wasn't Pixie who ended up Wolverine's new sidekick, it was Armor, also a young girl.
Inverted in the case of X-23, whom he least tries to edge away from utter sociopathy and hoped to secure a more-or-less ordinary life for before Summers decided she would be a good addition to the covert wetwork squad he was putting together.
Oddly enough, X-23 often acts as a Morality Pet for Emma Frost. And has a Morality Pet of her own in the form of Hellion. And now she's acting as a morality pet for Gambit.
Subverted: Harley Quinn, The Joker's sidekick, is just as insane as he is.
Harley has her Hyenas too.
The easiest way of making the Joker look like a complete and utter bastard is for him to betray Harley. Harley does have a stopping point when the Joker gets too out of control. Her betrayal of him is much more forgiven than his betrayals of her (which tend to be very violent).
She has also acted as one to Poison Ivy. Whether or not Harley is around often determines whether Ivy is a Well-Intentioned Extremist serial killer, or merely Harley's snarky and more level-headed Cool Big Sis. No Man's Land also gave Ivy over a dozen morality pets in the form of a group of orphans she takes in and cares for in the quake ravaged city. When she surrenders to the police rather than let one of them die, even Batman admits that despite her claims to the contrary, Ivy is still a human being at heart.
In James Robinson's Starman, The Shade (already ret-conned from villain to just plain Anti-Hero) has a certain respect for Jack "Starman" Knight and falls in love with police officer Hope O'Dare. His true Morality Pet, though, is his home, Opal City; it is mainly because of his vow never to commit crimes in Opal City that he is able to make friends with the local law enforcement.
In the IDW Transformers comic, Sixshot is a war machine who doesn't think twice when instructed to wipe out entire civilizations, and was a prime candidate for membership with the planet-euthanizing Reapers. But there's nothing in this universe that would make him hurt his buddies/fan club the Terrorcons.
On a similar note Runamuck and Runabout are two thugs, basically only in the Decepticon army because they get to carry a gun and cause some damage. But if you hurt one in front of the other you're gonna be in trouble.
J. Jonah Jameson from Spider-Man is more of a cranky, cigar-chomping foil than a villain (these days), but Robbie Robertson acts pretty well as his conscience, and you can count on seeing his softer side whenever his astronaut son John shows up.
Harry Osborn was occasionally this toward his father Norman (aka the Green Goblin); in one storyline, Spiderman defeated the Goblin by showing him his son in the hospital after a drug overdose.
Todd "Squee" Casil is the involuntary morality pet of Johnny the Homicidal Maniac, who sneaks into his house for medical supplies and once saved Squee from a pedophile. While Squee is scared shitless of Johnny (calling him the "scary neighbor man"), Johnny tends to see himself as something of father figure to Squee, dispensing aesops because he doesn't want Squee to end up like he did.
It didn't last long, and she didn't make him turn good or consider turning good, but the appearance of Loki's mortal daughter Tess Black in the Spider-Man comic "The Coming of Chaos" made the God of Mischief look less like a Magnificent Bastard.
In the Ranma, and other thingsFan FicHybrid Theory. Akane is a subversion of this. She and Chris think that his promise to her is keeping him from descending into true villain-hood, but it doesn't end up meaning much at all.
Adamska's love for Otacon and his alternate-future self's love for Big Boss in Stray.
In the Star Wars fic The Lost Child, even after turning to the Dark Side, Anakin still cares about Ahsoka enough to let her live.
The Star Trek fic Safe and Sound has Lucille Harewood (the little girl in the opening of the second film) be this for Khan. She is able to stop him from killing the man who killed most of his crew, and by the end of the story, he decides to accept Starfleet's offer of rehabilitation for her sake.
Not that Sulley was in any way villainous before he met her, but her reaction to him roaring doesclue him in on how traumatic his job can be for the kids he scares.
Yubaba's son Buu plays this role in Spirited Away, being the only person she cares more for than money. In fact, when Haku implied that he had taken Buu away (when he didn't), she went full Mama Bear on him to find out where her baby was.
Chihiro is this herself, able to bring the best out of the grumpy Lin and Kamaji and the cold Haku and geniunely befriends No-Face.
In Care BearsMovie II,Dark Heartmanipulates a girl named Christy into helping him, but develops affection for her after she saves him from drowning. After accidentally harming her, his remorse is enough to not only help save her, but also turn him human.
In Tangled, Rapunzel becomes this for Flynn Rider. While he's not an evil guy, he still starts off showing very little concern for his cohorts (he abandons his previous two partners in crime to be arrested and, likely, executed). When he ends up dragging Rapunzel into dangerous situations though, he does whatever he can to take care of her, and ends up falling in love and dying to set her free from Mother Gothel.
In Treasure Planet, Long John Silver has a literal Morality Pet called Morph, a cute bloblike alien that apparently represents his conscience. One could argue Jim Hawkins himself is another Morality Pet for Silver.
Mass murderer Riddick managed to find a few morality pets in Pitch Black, Fry, Jack, and Imam. Of course, Fry died at the end of Pitch Black, and Jack (Kyra) and Imam died at the end of The Chronicles of Riddick, leaving Riddick with no more morality pets, and a whole empire of necromongers at his command! Be afraid. Be very afraid.
Averted in Sweeney Todd: The Demon Barber of Fleet Street. Mrs. Lovett seems to have a soft spot for Toby and treats him as a son, while trying to keep him sheltered from her and the title character's business of murdering people and baking them into pies. Despite this, when he catches on to Sweeney's murdering ways (through his discovery of a purse belonging to Pirelli, his last master and Sweeney's very first victim, in Mrs. Lovett's possession), Mrs. Lovett's first instinct is to lock the poor kid into the basement where the pies are made preparatory to having Sweeney murder him. To top it all off she's also been feeding him human meat without him ever realizing it.
Maybe played straighter with Sweeney and Anthony. He's a non-threatening, innocent young character, whose puppy love for Johanna might reflect the young Sweeney's for Lucy.
Harley, the Tennessee boy who helps Iron Man repair and recharge his armour, in Iron Man 3 is this to Tony Stark.
How the Grinch Stole Christmas! has Max be a literal one. Although he participates in his master's evil deeds, he makes the Grinch save Cindy from the machine in the Post Office. He even has his own Christmas party while the Grinch is down in Whoville... until his master interrupts him and throws him out.
Inverted in Duumvirate. Luke is Paul's Immorality Pet, there to let him know that brutality, murder, and full-scale genocide are perfectly all right.
In the Dragonlance novels, the ambitious young wizard Raistlin Majere (who goes from Tall, Dark and Snarky party member and Ensemble Dark Horse to full-on A God Am IBig Bad between series...) has an early Pet the Dog moment with the downtrodden gully-dwarf Bupu, whom he unfailingly treats with kindness despite the cruelty and ridicule her race is treated with on general principle. She becomes symbolic of his last ties to humanity and shows up several more times in his life, including a heartbreaking postmortem appearance in Raistlin's doomed, ash-choked future world.
In the Chalet School series, the Robin acts as one to Jo, and later, to some of the more difficult girls, such as Gwensi Howell (The Chalet School at War) and Zephyr Burthill (Jo To The Rescue). When we first meet her, she's a cute Ill Girl with an exotic accent and a dead mother. Practically everyone likes her, and being mean to her is considered an act of kicking the dog. She even becomes a nun later on in the series.
In the Discworld novels by Terry Pratchett, Captain Carrot's first appearance (in Guards! Guards!) would count as being a Morality Pet for the Night Watch—he is everything they are not and, in many ways, are supposed to be. Even his naivete serves him well, in Ankh-Morpork of all cities.
Similarly, Lord Vetinari might be said to have a group of Morality Pets in the forms of his favorite catspaws: Vimes, von Lipwig, etc. A more literal Morality Pet might be the Patrician's since Making Money, late wire-hair terrier Wuffles, rumored to be the only living thing he cares for, and whose grave he still visits regularly. A more metaphorical Morality Pet might be the city itself, which he treats with care and even affection, striving to improve it, to expand its influence and power, to modernize it—but always, and most importantly, to keep it running.
However, note that neither the Night Watch nor Vetinari are "villains" per se. (Though Vetinari thinks he is a villian, but that's because, as we learn from his Breaking Speech in Guards! Guards!, he thinks everyone is a villain. In Vetinari's world, there are only evil people, but some of them are on different sides. He's on the side of Ankh-Morpork.)
A version is Death's adopted daughter Ysabell which humanizes him, but inverted in that, while Death is shown as reasonable, in her first appearance in The Light Fantasic Ysabell is literally Ax-Crazy.
Reversed with Death's granddaughter Susan. She was portrayed as rather cold in her first appearance, and it was arguably interacting with Death that humanized her (at the end she even allows herself to cry over her parents' death, something she had refused to do for years). Her subsequent appearances have had her working with children (as a governess and later a teacher), and these jobs seem to have increased her humanity further.
In the Gone series, Diana for Caine, although their relationship does show some abusive patterns on both sides. In book four, he Took a Level in Jerkass and drove her away
One of Forgotten Realms' old villains, the Zulkir of Enchantment Lauzoril was a Magnificent Bastard leading the Imperialist (pro-conquest) party of Thay. In The Simbul's Gift he turns out to be caring for his wife and particularly daughter, up to the point of taking an oath that his magic will never touch her, ever. note Dubious idea, but understandable considering what was done to his wife "for her own good" by her father. Lauzoril killed him, but could not undo damage.On behalf of his daughter he eagerly breaks not only Thayan law, but rules he set for himself. And risks his life.
Artemis Entreri and Jarlaxle Baenre act as each other's morality pet, in a sense, as their friendship shows that they are capable of such. (It's especially intriguing when one considers Jarlaxle seems to be trying to reform Artemis.) Artemis also has a halfling friend in his home city, whom he respects and, to some degree, trusts. Note that Jarlaxle isn't actually evil: Artemis is less his Morality Pet and more his 'Look, he's capable of forming actual positive emotions right there in front of us' Pet.
The humble scholarship boy Redwing to the unscrupulous millionaire's son Vernon-Smith in the Greyfriars series. A slight variation in that while Redwing's steadying influence generally helps Smithy stay on the straight-and-narrow (well, roughly), when Smithy flat-out means to be bad, Redwing's intervention is met with a hypersensitive hostile backlash that can make things worse. Later in their friendship (for example, the Smedley series) Redwing learns not to waste a 'pi-jaw' on Smithy, and simply waits anxiously for the moment to beat some sense into him.
In The Vampire Tapestry by Suzy McKee Charnas, psychologist Floria Landauer functions as this for the vampire, Weyland, in that he reveals himself honestly to her, will not harm her, and even, for her sake, refrains from killing one of her clients who is harassing him.
Brienne for Jaime Lannister. After his Heel-Face Turn, Jaime actually pretty much gathers up a Morality Menagerie - Pia, his squires, Tyrion, Tommen, and Myrcella all inspire feelings of devotion and affection in him.
Tyrion in turn has Jon Snow and Bran. His early interactions with them demonstrate that he isn't such a bad guy. Later, he grabs fellow dwarf Penny.
Both Stark sisters play this role to Sandow Clegane at different points.
Davos is Stannis's Morality Pet. Their relationship is one of the only really likable things about Stannis.
In an interesting twist, Lucius, Narcissa and Draco Malfoy all seem to play this role for each other in Harry Potter, most particularly when Narcissa helps Harry fake his death, just so she can go back to Hogwarts and find Draco.
In a weird, squicky case, one could argue Nagini serves this role for Voldemort. Granted, Nagini's evil too...but still, she seems to be the only creature he shows any true affection for, as messed up as that is.
Depending how you look at it, Voldemort is either Bellatrix's morality pet or a subversion, since her love for him only makes her moreAx-Crazy.
Kreacher seems to have filled this role for Regulus (or, for that matter, Regulus' memory for Kreacher). Really the whole Black family, since Hermione speculates his secrecy was explicitly to protect them from Voldemort's wrath.
In I, Claudius, Tiberius is portrayed as a pedophile who murders most of his relatives and a good chunk of the senate but for some reason he insists on having an innocent and virtuous senator live with him in his Evil Playboy Mansion on Capri. When the senator decides to commit suicide Tiberius is distraught, and actually goes so far as to tear up some death warrants in the hope that this will convince the senator to live on. (It doesn't work: the man wasn't committing suicide to protest Tiberius's crimes, but because he was tired of life.)
In one of the tie-in novels for Werewolf: The Apocalypse, Well-Intentioned Extremist Arkady sets out to attack Malfeas, encounters a little girl who's held captive by Black Spiral Dancers, and spends some time first trying to get her out of the way and then trying to protect her as she follows him. She's Really 700 Years Old and a manifestation of the White Howler totem.
Subverted in The Death of the Vazir Mukhtar, where Griboyedov tries to help out ensign Vishnyakov, but ultimately fails (and what's more, the reason for it might be that his superiors, on whose decision Vishnyakov's fate depended, were somewhat antagonised beforehand by Griboyedov's aggressive lobbying of one of his projects), and Vishnyakov blames him for everything and commits suicide.
Lonely Werewolf Girl is one big book full of Black and Gray Morality. The darkest gray character of all is probably the most probably the title character, Kalix MacRinnalch. As of the second book, she's trying (Well, maybe trying isn't quite the word-being forced is more accurate) to go to remedial school. What's the only thing keeping her from quitting completely, in spite of all the werewolves trying to kill her, her anorexia and angstiness? Surprisingly enough, it's Vex (Short for Agrivex.) who keeps her from dropping out, admitting that she pretends college is fun to inspire Kalix.
In Dean Koontz' Frankenstein series, Mr. Lyss is humanized by the time he spends with Nummy, who he admits is the only person he's ever met that he doesn't hate. At the end he visits the grave of the grandmother who raised Nummy, whom Lyss had never met, and thanks her for the greatest gift he's ever received.
Arguably, in P.G. Wodehouse's stories, Bertie Wooster plays this role with Jeeves (even though Bertie is nominally the boss). In the one Jeeves novel where Wooster does not appear, Jeeves shows no compunction about taking part in crime, and in the one story Jeeves narrates, he hints that he has had no reservations about serving criminal masters in the past. But while he's with the (stupid and ineffectual but lovable) Bertie, his awesome abilities are used for good alone.
Clary for Jace in The Mortal Instruments - The reason he didn't go with Valentine through the Portal in City of Bones was because he was thinking of Clary.
By the time of Star Trek: The Fall, Dr. Parmak is Garak's Morality Pet. He openly refers to Parmak as his conscience, and seeks his presence when wrestling with his questionable personal history.
Michael's mother and to a lesser extent his brother are his morality pets. His mother is a straight example. Nate is more complicated as he is useless and morally dubious but harmless and he frustrates Michael a lot. However the fact that he awakens Michael's Big Brother Instinct makes him this.
Michael's more pitiful clients tend to be this for Michael as well.
Michael is arguably Fiona's Morality Pet. She would be a lot nastier without him.
Charmed: Chris is hinted to be this for evil Wyatt.
This is rather ironic as in the first season finale, Jeff ultimately chooses Annie as a representation of both change and being oneself in comparison to Britta and Slater. Quite literally, Annie was the best of both worlds for Jeff.
Also Pierce; Pierce has on several occasions noted that Annie is his favourite, and if anyone is likely to provoke a glimpse into his extremely well-buried nice side, it's her. Conversely, Annie will also be the first to defend Pierce to the others.
Criminal Minds: In a very dark example, Ellie Spicer becomes this to Billy Flynn, the most prolific serial killer and rapist to appear on the show. Killing Ellie's grandparents when her father, Matt, was a child, Billy develops a "grandfather complex" towards Ellie. Killing Matt, Billy kidnaps Ellie and tries to turn her into a protege, treating her as the granddaughter he doesn't have. Sick as it may be, his treatment of Ellie is the only evidence that there might still be something human inside Billy; when the fact that what he's doing to her is bad is pointed out to him, Billy lets her go, than goes to his suicidal death.
The new series has suggested that the Doctor needs his companions to be this for him. After he loses Rose and Donna initially decides not to go with him, she tells him to find someone because sometimes he "needs someone to stop [him]" from going too far.
As SF Debris noted, this was possibly apparent from his earliest appearance in the Classic Series, when the First Doctor was ready to bash an injured man's head in to save his own hide and only stopped by the intervention by the human he'd previously disparaged for being a primitive in comparison to himself.
It's also worth noting that The Deadly Assassin veered into darker territory with the Fourth Doctor, which took place immediately after the departure of Sarah-Jane Smith and wherein the Doctor had no companion for the duration of the story.
Firefly: Not a single member of the crew is too tough to be concerned about Kaylee.
Sue's sister, and Becky Jackson. It's kind played with with Becky, as Sue is about as abusive to Becky as she is to other students, which is kind of the point, a she has Down's Syndrome, and Sue treats her like any other girl her age, likely because of her sister
Beth, daughter of Quinn and Puck, though she's more Puck's morality pet.
Brittany, for Santana, showing that the latter is actually capable of a deep interpersonal relationship.
Rachel for Jesse. He's a dick to pretty much everyone except Rachel.
A meta example - Ryan Murphy's other shows (Nip/Tuck, American Horror Story) are intensely dark and disturbing. If you buy Glee's hopeful and optimistic message of inclusiveness as sincere, the show itself becomes this.
In the fourth episode of season 3, a kid named Noah, who is apparently Sylar's son is the most distilled version of the Morality Pet ever seen for future!Sylar. He claims to have curbed his "hunger" for him. When he dies, Sylar explodes.
This show has quite a few: Nikki has a son that she throws herself into the action for to protect. More significantly, HRG has a surrogate daughter which leads him toward a straighter path than if he didn't have a daughter.
Heroes writers apparently believe Sylar is in desperate need of a Morality Pet. At the end of Season 4, Peter arguably applies, when he and Sylar are trapped together in the episode, 'the Wall', although it could be argued that Peter wasn't the impetus for Sylar's change of heart; Sylar's reformation was already underway. Too bad, but I think I see some Badass Decay in Sylar's future.
Whatever else Nathan Petrelli may be, you can't say he doesn't love his little brother.
The writers tried to set up Cameron as one in the earlier seasons, but it didn't really take and she eventually turned into a self-righteous hypocrite who only thinks she's a Morality Pet.
Jericho: In season 2, Heather is a textbook example of that to Major Beck.
Kamen Rider Ryuki: Takeshi saves a little girl and protects her from monsters in an apparent Pet the Dog moment. It turns into a Kick the Dog when Asakura reveals that he cares nothing for her and was just using her as monster bait. Then back to a Pet the Dog when he ends up waiting outside her hospital bedroom to see if she gets better, even after she has no more use to him.
Charles' little sister Honoria appears to be one to him. He is a complete pompous jerkass to everybody else but his love for her inspires him to stick up for a soldier who stutters. Later, it is revealed Honoria stutters. The few times we see him addressing her via tape recording, he speaks in a much gentler, softer voice than to anybody else.
BJ is frequently this to Hawkeye as well, in contrast to Trapper who was purely The Foil. This is subverted in a few episodes in that 1) Hawkeye doesn't always listen to him and 2) BJ can be in need of a restraint sometimes himself (such as when Hawkeye has to stop him from punching out a patient in "Rally 'Round the Flagg, Boys" or when BJ has an emotional breakdown in "Period of Adjustment.")
Also subverted with Margaret and Frank in that, though Margaret is the only person (besides Frank's mother) he is shown to care about, he is just as terrible and nasty with her as he without, though possibly a little saner.
Oddly averted with Radar. You'd think he would be one of these given his loveable farmboy-naif personality, but the various jerkass/antagonist characters (Frank, Charles, Col. Flagg) not only consistently treat him like crap but seem to make an express point of doing so. Even Margaret never quite warmed up to him like she eventually did with Hawkeye and co.
NCIS: Gibbs is all about business and tough love with almost everyone, except with Abby, whom he kisses on the cheek and gives hugs, massages and occasional roses, among other things. No, they're not screwing. It's a father/daughter relationship. He also acts significantly warmer towards children (stated to be because "when they lie, they don't have the guile to get away with it") and his father.
Once Upon a Time: Henry is this for Regina/The Evil Queen. In season two, Belle and his son, Baelfire/Neal is this for Gold/Rumplestiltskin. In season three, Henry also becomes this for Rumplestiltskin.
Orphan Black: Kira serves as this for Helena. It's implied she plays a similar role for her mother, Sarah.
Power Rangers: Several villains in have morality pets that show them capable of love and kindness and may even try (intentionally or not) to cause a Heel-Face Turn, but the only one who actually succeeded was Nadira from Power Rangers Time Force, daughter of the Big Bad, Ransik. He showed during the series that he was willing to put his hatred aside for her happiness, and pulled a Heel-Face Turn entirely when she jumped into the line of fire after twigging onto the Cycle of Revenge.
Revolution: Emma Bennett in the episode "Home" acts as this to Sebastian Monroe of all people. While she wasn't able to get him to stop trying to kill people, he did prove that there is a measure of humanity left in him and that he doesn't enjoy being a Big Bad guy. Her death really got to him, and the fact that she had given birth to his son and sent him away some time ago eventually causes him to experience a Heel Realization.
Rome: Has Eirene, Titus Pullo's vulnerable, doe-eyed slave and later love interest, whom he rescued. Unfortunately, Pullo's a Sociopathic Hero (though played for drama more than comedy) and his idea of expressing love for her is a little...warped.
Dr. Cox and Jordan seem to have a morality pet in their son, Jack and their daughter, Jennifer Dylan. The main character, J.D. might also count as one.
In a strange way, Dr. Cox is something of a Morality Pet for J.D. Rather than in the traditional sense, J.D.'s interactions with his mentor sometimes force him to really think about whether he's more interested in playing by the rules or saving lives, and which is the "right" choice when the two diverge.
The Janitor has one in Elliot (or "Blonde Doctor" as he would call her) in the more traditional sense.
Also, Adam for Cassie. Adam prevents Cassie from going over the edge, especially in regards to her Dark Magic. Adam, along with Diana, is one of the few people who can actually get through to Cassie.
Diana for Charles.
Cassie for Jake.
Melissa for Nick.
The Shadow Line: Has Joseph Bede's Alzheimer's-afflicted wife. It makes it a lot easier to root for a wholesale cocaine dealer knowing that's he's only involved in One Last Job to pay for her treatment.
John Watson for Sherlock. Sherlock sees him as a friend; indeed, John is the only person Sherlock sees as a friend. John's presence humanizes Sherlock somewhat, forcing into question Sherlock's self-diagnosis of "high-functioning sociopath". When John's life is threatened in 'The Great Game', solving what Moriarty is up to becomes second to making sure John is all right. Moriarty even refers to John as Sherlock's "little pet".
Also, Mrs Hudson - Sherlock may not count her as a friend, per se, but he is openly affectionate with her in a way he never is with anyone who is not John.
Skins: Effy was this for Tony, since his relationship with her was the only sign that he had any empathy at all during the first season. Although, Effy was much more than a Morality Pet and perfectly capable of carrying her own stories, so this example isn't one-dimensional as the trope description might suggest.
For a while, Grant Gabriel/Julian Luthor 3.0, who Lex personally arranged to become editor of the Daily Planet, was regularly invited to his mansion so they could have a brotherly relationship. Then, when Grant learns that he's really a clone of Lex's dead brother and tries to live his own life and going against Lex's plans, he has him killed in a supposed failed mugging.
Prior to both, Clark served as one, and we all know how well that goes.
Stargate SG-1: Daniel Jackson and to some extent the whole team for Jack O'Neill. Daniel, enthusiastic, morally upright, slightly gullible civilian, seems to bring out the nobler side of the grouchy, hardened, ex-black ops O'Neill, who can be pretty ruthless when left to his own devices. Though the stances were reversed on one occasion.
Star Trek: Deep Space Nine: Tora Ziyal is this to her Cardassian father, Gul Dukat during seasons 4, 5, and 6. Though since Dukat goes insane after she is killed and eventually becomes the Bajoran antichrist, this could also be a Morality Chain.
Gives evil vampire Klaus one in the form of Caroline.
Elena is this for Damon. On occasions Stefan, and lately Jeremy fulfill this function but it's really all about Elena. Depending on the viewer's interpretation of Damon, Elena may in fact be his Morality Chain.
In WWE, long-running Foreign Wrestling HeelWilliam Regal was assigned by Eric Bischoff to baby-sit and team with his "special" nephew Eugene, a mentally-challenged man who was, despite his handicap, a capable wrestler. Regal quickly grew fond of Eugene, to the point where Triple H's cold, cynical manipulation of Eugene served as a catalyst for a rare Heel-Face Turn for Regal.
In The Elysium Project, Shay acts as this towards Mirage after their time together at Highmore Academy.
Barnabas was this to Saul/Paul when the other disciples questioned Saul's Heel-Face Turn. See Acts 9.
Archangel Dominic in In Nomine is often depicted as an insane Knight Templar. One person's rather popular fanon solution to this was to give him a kitten to soften his image. This more or less worked for a large portion of the fandom.
Subverted in Children of the Night, a supplement for Vampire: The Masquerade. In it is a character profile of Velya the Vivisectionist, who fell in love with a 10-year old girl and made her into a vampire. The source of his infatuation with her was her blatant sociopathy, and willingness to manipulate her family into insanity and murder. But that's not the worst part. Even though she was as evil as they came, Elaine Cassidy wasn't mentally equipped to handle vampiric life beside one of the most cold and calculating inhumans in the world, and quickly went batshit insane. Velya's response to his "wife's" degeneration was to graft her body into his torso in order to keep her out of trouble until such time as he could find a cure.
The eponymous character in Verdi's opera Rigoletto will simply be a small-minded, vengeful court jester, if not for his daughter Gilda.
Dragon Quest IV Gives Psaro one in the form of Rose. Her influence is the only thing that keeps him from completely and utterly wiping out the human race. The second she's killed, he loses his mind and initiates a transformation so that he can destroy all mankind in retaliation. In the remakes, one can bring Rose back to life with through a sidequest. Bringing her to the location of the final boss fight causes Psaro to regain his sanity, transform back, and join the party to exact revenge on The Dragon for planning her death.
Dragon Quest IX plays with this in Coffinwell: Doctor Phlegming is very antisocial, and his cheerful wife Catarrhina appears to be the only other person he cares about... yet he still neglects her in favor of his research. In fact, it's actually her death that spurs him to change his ways for the better, effectively Inverting the usual trope.
In the H-GameIdols Galore, the protagonist would otherwise be a Jerk Ass at best, if it wasn't revealed that it's mostly a Jerkass Façade (albeit to hide his very reluctant good qualities). In fact, if he gets the ending with his Morality Pet, its revealed he's actually a fairly decent and even noble person who was hiding under several layers of asshole because he didn't want his inner decency to show.
In a city of twelve million people, there's only one person Alex Mercer gives a damn about: his sister, Dana. In fact, ultimately it turns out that the Blacklight Virus cares more about her than the real Alex Mercer ever did.
Arguably, Melissa Woodward, in Hotel Dusk: Room 215: Room 215. Main character Kyle Hyde normally looks, sounds, and acts like a washed-up Film Noir detective, but his actions with Melissa (including acting as her nanny, getting her out of a locked room 20 minutes later, giving her a stuffed rabbit doll, and getting an entire Christmas tree out of storage because she was complaining she didn't get anything for Christmas) prompts Hyde to note that he acts like "a sap".
The Crusader games don't really show how the Silencer reacts around his fellow Rebels, but the fact that he's apparently willing to stop by the bar and talk with them, or at least listen to them, makes for a nice contrast with his being a sociopathic killing machine.
In Betrayal at Krondor, whenever the dark elf Gorath does something that shows off his softer side, it usually involves his plucky young human companion Owyn.
Most BioWare games with romance plots give you Good, Paragon, Open Palm, or Light Side points for doing things that advance the romance. So, to play an evil character with a Love Interest, you have to turn back from the Dark Side at least a little bit, making Aribeth, Aarin, Ashley, Kaidan, Liara, Dawn Star, Sky, Carth, Bastila, Ashara, Mako, Vette and all the others all count as Morality Pets for the player.
This effect is noticeably absent in cases where the Love Interest is at least somewhat evil; see the Closed Fist option with Silk Fox, the dark side option with Bastila, and Evil!Aribeth. The Male Sith Warrior and a corrupted Jaesa is also a very twisted case of Unholy Matrimony, as is a female Inquisitor and the pirate captain Revel. It's also subverted with some of the Republic classes (especially the Jedi Knight, the male Jedi Consular, and the Republic Trooper), as romantic options can actually net you Dark Side points.
Merrill in Dragon Age II serves as this for most of the cast; no matter how stoic, arrogant, or generally jerkish the character, everyone's got a soft spot for Merrill.
It doesn't even have to be a Romance Sidequest. Some characters, like Merill (above), Areie, Deekin, Blizz, Wild Flower, Mission Vao, Bethany (or Carver) tend to bring out Big Brother Instinct or Mama Bear reactions instead. They're just as good at putting the brakes on a dark-side action because you'd rather be a good example.
In the later missions of StarCraft, Jim Raynor might be seen as the Morality Pet for Tassadar and the other Aiur-dwelling Protoss.
Matt Horner serves as one for Raynor in the sequel, stopping him from getting carried away.
Subverted in Baldur's Gate II: Irenicus tries to use his old lover as a Morality Pet, but since he's been exiled and stripped of emotional response (aside from thundering rages), every attempt he makes ends up being both misguided and just plain creepy.
To wit: he clones her, but the clones are insane and/or desperate not to be controlled by him (he can't actually feel anything for them, after all); he kidnaps dryads in an attempt to kindle the fires of passion, but then treats them more like (implied) sex slaves; he strips out the soul of a godling in an attempt to replace his own. And through all that, when he finally gets the chance at redemption and love with his old lover, he's so far gone down his path that he opts instead to try to kill her.
In most single-player video- or computer-game RPGs, NPC party members aren't so much Morality Pet as they are Morality Compass - good characters will approve of good actions and deride evil ones, and vice-versa - but the fact that, for example, the option remains for the Player Character's old friend from childhood to still be treated like a favorite sister even after they've taken down a kidnapping ring and set themselves up in its place probably makes many players feel warm and fuzzy inside.
Averted in Black & White: the player's pet is a representation of the player and acts according to the player's desires. (Well, most of the time.) Of course, the player's desires can include eating pooping all over one's followers?
Raphael from the Soul Calibur series would seem more like a villain instead of an anti-hero if it wasn't for his foster daughter, Amy, being the driving motivation behind his actions.
Of course, these actions include trying to start a bloody civil war, and later trying to turn the entire world into vampires. This may be more an example of Love Makes You Evil.
Volgin's care for his lover, Raikov, in Metal Gear Solid 3. It's pretty much his only redeeming feature. The End also has his own Morality Pet, his pet parrot. If you shoot it, he will make a lot of noise about it. And heaven help you if you eat it...
Snake pours scorn on the very idea, and mocks Otacon for believing that liking dogs proves that Wolf is a good person.
In Fire Emblem: Sword of Flame, Nino is Jaffar's Morality Pet, changing him from a soulless Angel of Death to someone with actual feelings.
Lesser example, in Path of Radiance, the main character Ike is very much one for Soren, who has a tendency to be amoral and callous at the best of times. Played with in that Ike's the big, imposing one.
In No More Heroes, Travis owns a kitten called Jeane that you can feed and play around with. This has no gameplay purpose except to show that Travis isn't such a bad guy.
In the sequel, helping your (now) overweight cat get back into shape unlocks a special move.
Final Fantasy VI: Shadow's devotion to his dog Interceptor is one of the chief things that humanizes him, despite the fact that the dog is a rabid killing machine that, according to his owner, feeds on strangers.
Although the "feeds on strangers" thing could be a lie. Oh, and Relm is this for Interceptor! (Although if you see all the dreams, then you know why, which also makes her one for Shadow!)
Which he then subverts by promising her that if it wasn't for the way she looks, he would have murdered her a long time ago for the hell of it.
Kel'Thuzad, The Dragon to The Lich King has a little kitten within his Boss Room named Mr. Bigglesworth. If players kill it, he'll let out a Big "NO!" while swearing that his armies of undead will hunt you down. Officially, it's the last of his humanity. So it really is the closest thing he has to not being pure evil.
Cloud himself is a Morality Pet for the majority of the cast, since he's gone through so much and keeps the cast going even when things seem hopeless.
In Gungrave, player character Grave's young protectee Mika is one the few things that keeps him from being a completely mindless killer, as she is a memento of the people he held near and dear when he was still Brandon Heat. She is something he can "live" for, the one good thing left in his "life".
In Quest For Glory IV: Shadows of DarknessTanya fills this role towards Katrina, who wants to resurrect the local Eldritch Abomination. Boy, do you feel like a jerk when do the right thing by returning Tanya to her rightful parents. Fortunately for you, by the time this happens in the game, you've become Katrina's new morality pet, even though she doesn't realize it until it's too late.
Harvest Moon DS has Daryl, the creepy Mad Scientist living in the southern part of the valley. In previous games, he was known for wanting to experiment on your cows and accusingly discussed the effects of methane produced by cows on global warming. In this game, he stalks archaeologist Flora. However, if you decide that you actually want to befriend him... You discover that he rescued an injured mermaid girl named Leia after she washed up on the beach, and has been diligently caring for her ever since. She's so grateful to him, she helps him cook, and despite her being an anomaly of science, he never once experiments on her. In fact, when she's all better, he sets her free into the ocean, and she's forever grateful.
The Magypsies in MOTHER 3 all have pets to show their caring side, but Locria's pet mouse is the best example, being the only hint that he wasn't totally evil.
Fanon likes to make Adrian Andrews into a Morality Pet for Franziska Von Karma. In the actual games, she does latch onto Von Karma because of her dependency issues, but the main reason Von Karma is any nicer to Andrews than she is to anyone else is that at this point in continuity, she's still dedicated to her win records and knows that this is the best way to get Andrews to work with her in court. Later on, when Von Karma is less of a royal bitch and doesn't need a Morality Pet so much, Andrews is learning to stand on her own, without the aid of the strong mentor figures she relied on.
To some extent Trucy acts as a sort of Morality Pet for Phoenix after he is disbarred, and as we see that he becomes totally Bad Ass after being disbarred, if he hadn't had a Morality Pet...
Mia and later Maya as a Morality Pet for Godot. It's his prime motivation for why he killed Elise Deauxnim, who had the spirit of Dahlia Hawthorne inhabiting her body. Maya even tries to take the heat for him.
In Pokémon Colosseum, ex-Team Snagem member Wes (also the main character) has Rui. He actually decided to flatten the first evil gang in the game before meeting her, having apparently decided that he just didn't like them anymore, but didn't decide to actually help save Orre until she showed up.
In Pokémon Ruby and Sapphire and Diamond and Pearl there is a subtle example. The criminal leaders of Team Aqua, Team Magma, and Team Galactic (Archie, Maxie, and Cyrus respectively) had a Golbat the first time you fight them. In the later encounters, they have a Crobat, a Pokemon which evolves from a Golbat that has reached maximum happiness with its trainer.
Cyrus raising a Crobat is pretty much the exact opposite of his Straw Nihilist philosophies, emphasizing its Morality Pet status due to the hypocrisy.
Wes also has an Espeon and Umbreon at the very beginning of the game, both of whom evolve through happiness like Crobat.
Even Kratos of God of War has one in his daughter Calliope. While for the most part a Sociopathic Hero, invoking his Papa Wolf nature can lead him into acts of heroism and selflessness, as seen in Chains of Olympus, where he gives up the chance to remain with his beloved daughter in the Elysium Fields in order to save her and the world, including a button-mashing sequence where he must push her away.
In the third game, he gains another one in the form of Pandora.
One of the more shocking twists in Ghost Trick is that Big Bad Yomiel had one of these...and it's the main character. Sissel's Ghost Amnesia made him forget that he was a stray cat who became Yomiel's sole companion during ten years of undead isolation from humanity.
Theresia: Dear Emile subverts this. Emile killed Leanne's mother when the latter was an infant, but kept her alive and raised her in an attempt to understand the concept of love. Seeming more obsessed than truly maternal, she kept the child isolated from all other people, and sometimes kept her in chains. She never showed any love or affection towards her, but Leanne loved her anyway and considered her a mother figure, growing to love the smell of blood and the sound of screaming as reminders of her. Ultimately, Emile protects Leanne from the other soldiers, but tries to kill her herself rather than let her escape to the outside world.
Taro acts as one to Rozalin in Disgaea 2, and Etna's a lot nicer to Hanako then she is to the rest of the party.
Raspberyl to Mao in the third game. Becomes a Morality Chain in the bad ending.
Flonne to both Laharl and Enta (again) in the 1st game.
Amy Rose from Sonic the Hedgehog towards E-102 Gamma, Silver, and to a lesser extent Shadow (heavy influence on his Heel-Face Turn due to reminding him of Maria).
Portal 2 gives us Cave Johnson, the CEO of Aperture Science and his assistant Caroline. Cave may have been hilariously insane and disregarding of his employees' safeties, but he had lots of respect for his assistant, who were one of the few people who could restrain him when he lost his temper. While he eventually forced her to be uploaded unto GLaDOS later on against her will, it is still obvious he still has some ounce of respect for her, in that he does it not out of maliciousness, but due to wanting her to run the facility after his death due to his high regards of her.
Valve pulls this again in Team Fortress 2, giving The Medic a group of doves as pets in his "Meet The Medic" short. His favorite, Archimedes, appears to be just as morbid as he is, however...when things explode, he leans in to watch, often getting covered in blood, and he has a disturbing habit of nesting in patients' open chest cavities.
Although non-romantic and to a much lesser extent, part-squirrelPlucky Girl Makoto is also one for Jin. Jin snarks at her a lot, but he cares about her well-being, and saves her life when Hazama attempts to kill her.
In Street Fighter, Badass Grandpa Gen is one of the most merciless and skilled assassins in the world. However, there is one person whom he shows a somewhat softer side to: Chun Li, the daughter of an old friend of his's, as well as his favorite disciple. Further expanded in the UDON Comics, where one of the reasons why Gen stops being a hitman is having been forced to kill an opponent in front of a very young Chun-Li.
Sometimes, Sakura Kasugano is this to Ryu. The biggest example is in Alpha 3: after Sakura attempts a Go Through Me to protect a knocked-out Ryu from Bison and he just swats her away, Ryu temporarily goes berserk and tears Bison a new one. And this is after Ryu had been Brainwashed and Crazy.
She does it again (via Cooldown Hug) in Ties That Bind when Ryu loses himself to the Satsui no Hadou after Cammy is brutalized by C. Viper. It takes a crying, pleading Sakura to calm him back into sanity.
When the Creepy Twins Hsien-ko and Mei-Ling from Darkstalkers became Jiangshi, Hsien-ko was transformed into the body while Mei-Ling turned into the soul placed inside the warding talisman on her Nice Hat. If the hat ever was taken away and thus the twins were separated, a Hsien-ko without Mei-Ling's calming influence would turn straight-up evil.
In the What Did I Do to Deserve This, My Lord? series, Badmella, Badman's teenage daughter, is this for Badman. Both of the first two games include a mission where Badmella is deathly sick and Badman demands a remedy made of Slimmando Mosses in order to cure her, with his usual talkative and flippant demeanour gone in favour of frankness and desperation.
The Last of Us - Though not exactly inspiring Joel to greater heights of moral fibre, Ellie does give him a reason to live for, rather than simply survive.
Beyond: Two Souls: The protagonist, Jodie, acts as one for Aiden. When she was a little girl, the entity actually did many nice things for Jodie, including entertaining her with shadow puppets and fetching her a cookie from that out-of-reach cookie jar. Aiden also protects her using its powers. Unfortunately, the entity isn't so hospitable to other people. Usually, they never go beyond dickishbut otherwise harmless pranks, but if you harm or threaten Jodie, he can turn extremely nasty. Case in point: after Jodie is mistreated rather horribly by the attendants at a birthday party, Aiden locks them in a room and the retribution he can potentially reap onto them can include smashing windows in their faces as they frantically try to escape, throwing furniture at them, stabbing one of them with a kitchen knife, and even trying to burn the entire room. The bullies didpartly have it coming, but even then, this is a horrifically disproportionate response.
Tsugumi of Ever17 was probably coming off a little too strong on the jerkass and a little weak on the Broken Bird. Plus they needed a way to suddenly make her get along better with Takeshi, so they introduced Chami the Djungarian Hamster. He's named Chami because he's charming.
Not that Dr. Irie from Higurashi no Naku Koro ni was a villain, but Satako Houjou was an influence in changing some of his priorities.
H-GameGibo - Stepmother's Sin has the main charactter, Yusuke, doing pretty much everything he can to get into his stepmother's pants. However, he also has a stepsister, Shiina. In order to get the better endings (here defined as ones where you don't turn into a total bastard), you need to treat Shiina well, making her into one of these. Even when he's being a dick, Yusuke still refers to Shiina in kind terms.
Built in to troll psychology/romance on a fundamental level in Homestuck. MOIRALLEGIANCE pairs are basically this trope, where a calmer troll keeps a more hot-tempered or violent troll's dangerous tendencies in check and they balance and complement each other's personalities. Three examples exist at the beginning of the troll arc: Kanaya for Vriska, Feferi for Eridan, and Nepeta for Equius. Subverted in that Vriska is still a HUGE BITCH BLUH BLUH even with Kanaya's help, since Kanaya has less-platonic feelings for Vriska and therefore fails to exercise any real control over her, and Feferi broke up with Eridan early on because of the emotional toll of keeping him constantly in check. Nepeta plays it straight except for the fact that on closer examination Equius might not have been that bad a person without her anyways. (He says he might have been.)
The Equius <> Nepeta dynamic is also interesting as Equius believes that he is the one protecting Nepeta; later on, he admits conversely that, without Nepeta's support, he believes he would probably be violently unstable.
Kanaya also finds herself accidentally slipping into this role for Rose, but when Rose mentions it she tries to backpedal because she doesn't want a repeat of her relationship with Vriska.
As of late, Feferi dumping Eridan has had bad consequences for the team. On the flipside, Karkat became one of these for the psychoticGamzee, successfully ending his murder spree without any further bloodshed.
Subverted in Furmentation - Alexi goes into a violent, psychopathic rage when his teddy bear is stolen, but having it doesn't seem to make him any nicer than usual.
While he's antisocial rather than villainous, Davan Macintire and his relationship with Rory fit this trope. He's still grouchy and sarcastic around Rory, but he seems to give more of a damn about the kid than he does about most people even upon learning that Rory isn't actually his son.
Mr. Scruffy in The Order of the Stick, who is probably the only living thing Belkar genuinely likes and cares for besides himself. Mr. Scruffy doesn't seem to make a dent in Belkar's morality (or lack thereof) otherwise, though. In fact, Belkar seems to like Mr. Scruffy largely because the cat appears to be as much a vicious little bastard as Belkar himself (though he partially does it in honour of Shojo).
On the other hand, Belkar interprets such things as eating meat and batting at a piece of string as "vicious." ("Kick that string's ass.") He's inclined to see normal cat behavior as vicious and bloodthirsty; perhaps an unreliable interpretation of the cat's character.
That said, Mr. Scruffy also actively took part in Belkar's fight with the Thief's Guild, and a recent strip has him bat a bowl of maggots on a bunch of people to apparently escalate a bar fight...
Mr. Scruffy plays the role a little bit straighter here. Belkar even comments on that.
Belkar: Stupid cat. A ranger is supposed to influence an animal's behavior. You're doing it backwards.
Elan: I can't tell if the cat is a good influence on Belkar, or Belkar is a bad influence on the cat.
Hayley: Both, I think, but it probably still averages out somewhere south of Neutral.
Played much straighter with Blackwing to Vaarsuvius in the most recent arc. Vaarsuvius isn't so much evil as (possibly) True Neutral, but sie has anger problems.
While technically human, Piffany from Nodwick is the only reason Artax and Yeager actually display any kind of heroism ever. There's a reason A World Without Piffany is so crapsacky it makes even the normal Nodwick world seem pleasant.
Buck Godot has the Beemahs become this for the Klegdixal, starting in this comic.
Prior to, and unbeknownst to anyone else, the Beemahs were sort of the Morality Pet the Klegs wished they had-the meeting between the Beemah and the Kleg ambassador is kind of a Crowning Moment Of Heartwarming, as the usually rude and self-serving Klegdixal pours his heart out with gratitude to Buck for reuniting the two species, as the Klegs had apparently been the original creators of the Beemahs and their one selfless urge as a species was to fix all the things that had gone wrong in the process (many of them flaws which are used to exploit the Beemahs by another party).
Another side of it approaches Morality Chain, as 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.
Played with early in Sluggy Freelance, when Kiki the ferret's regular Catch Phrase was "Stay good, Riff! Stay good!" Otherwise averted; Torg's Immorality Pet doesn't seem to have changed him, and Bun-Bun's Alternate Reality Good Version didn't stop Torg from sliding into embittered self-interest in the Dimension of Lame.
In Accursed Dragon!, Wizard has Humpbrett, a strangely pet-like boy who has been referred to as a "stray" Wizard picked up.
Danya, Survival of the Fittest'sBig Bad, is occasionally revealed to take time off from his commitments running the Act (including making the announcements) and going home to see his family. The obvious love and affection he has for his wife, young daughter and grown-up son serve to humanise him somewhat. Most members, however, still loathe him.
They also fully support his activities, giving him even more motivation to act like a monster.
Also, to some extent, his then-fiancee Fox, when she temporarily becomes a werewolf.
Demona's daughter Angela. On any other occasion, Demona is still her usual self.
In the He-Man and She-Ra Christmas Special, Card-Carrying Villain Skeletor is chagrined to find himself looking after two Earth kids and their pet who wound up stuck on Eternia.
In Ben 10: Alien Force, the main reason Kevin decides to do anything good is because he has developed a crush on Gwen.
There's also Cash and JT: at the end of the episode, JT talks Cash out of letting the robot hand take over his body and fighting with Ben.
In the Samurai Jack episode "Tale of X9", the robot hitman X9, the only one of his model with free will and a processor about him, abandons his job after finding an adorable abandoned puppy, Lulu. As far as he was concerned, all he needed was her... which is why he was forced back to work when Aku kidnapped her. In a bit of a Tear Jerker, Jack cuts him down like any inconsequential enemy. X9 asks him to take care of Lulu, then dies.
On Total Drama,Duncan is a Jerkass to other characters most of the time, but finds his Morality Pet in Gwen and it's stayed that way for three seasons straight. He also is pretty good friends with Owen.
Subverted with Courtney. It started out playing this trope straight but Courtney's derailment and Duncan cheating on her, and being more of a jerk to her than anybody else changed all that.
Subverted in the Justice League episode featuring the Justice Lords. Flash is pretty sure that the Mirror Universe Superman (who's gone totalitarian nuts) won't be able to kill him because of what the Flash represented to that group. Reverse!Superman is quite prepared to heat-vision his brains out. His own universe's Superman admits that he knew even Flash wouldn't be able to talk Reverse!Superman out of it.
Flash: Can't do it, can you? I'm the last bit of your conscience. This is the one thing you'll never do.
Superman: I've done a lot of things I thought I'd never do these part two years. One more won't hurt.
The eponymous character of Jimmy Two-Shoes is this to Heloise since she's in love with him, but not even he's safe from her anger and wrath.
Without Jane Lane, the titular character of Daria would be, well, unpleasant to be around.
Also, as seen in the episode where Jane joins the Track team. Daria begins to obviously go mad, voicing her ENTIRE internal monologue.
A straighter example for Ice King is Marceline the Vampire Queen, who he was a Parental Substitute towards hundreds of years ago. While he's since gone completely insane, he still cares about her and she's noticeably the one person he actually cared when he believed he'd hurt her.
The Lemongrabs are this to Princess Bubblegum. Normally, she is very unforgiving of criminals and people who would dare lay a finger on her beloved subjects. But because she considers the earls to be her responsibility, even something like family, she will always forgive them, no matter how stupid, irrational, and bonkers they may be. The worst they could ever get from her is a verbal scolding and a slap on the wrist.
The Lemon Children seem to be this to the Lemongrabs. Normally the earls are rather rude and unpleasant, but they sincerely love their own children- perhaps even more than Princess Bubblegum loves them.
Taken to a dark degree with Young Justice: in the episode "Secrets," Harm killed his younger sister because she was the only person he cared about.
Harry to Norman Osborn in the Ultimate Spider-Man animated series. Unlike other adaptations, Norman Osborn does seem to care for his son and he shows genuine concern when he hears the Frightful Four are attacking his son's school. He's is still strict with Harry though.
Beast Wars, Rampage is a cannibalistic serial killer, akin to Hannibal Lecter as a transforming robot armed with mountain-busting weapons. He openly despises both sides of the conflict and would happily eat them all if he wasn't kept on a short leash by Megatron. However, he finds Transmutate, a horribly disfigured and mentally regressed Cybertronian whom the others view as a useless or even a liability, but as a freak like himself, he sees as a kindred spirit. When Transmutate dies, he actually mourns. As in "collapse onto the ground while weeping loudly" kind of mourn.
Moral Orel: Orel Puppington is this for his grandfather Arthur and possibly for Stephanie.
Darkwing Duck has his adopted daughter Gosalyn, who is probably the person D.W. loves the most other than himself.
Foster's Home for Imaginary Friends: Mr. Herriman has Madame Foster, his creator, he's normally no nonsense and uptight with pretty much everyone else in the house, including her granddaughter Frankie, but he truly loves Madame Foster (especially when you consider she never abandoned him after she grew up). He's even willing to perform the incredibly embarrassing "Funny Bunny" song for her.
In Garfield and Friends, in the U.S.Acres segment, in the later seasons, Roy Rooster has two Morality Pets. The first one his little niece, Chloe, he ditched his date to save her from the weasel and spent the rest of the night with her instead. The second one is Wade Duck sometimes. Altough he pranks him and they argue sometimes, Roy had moments of protective instincts towards Wade. In the "Snow Wade" two parter, Roy was the only character to show any concern for Wade's well being, when he didn't seem to wake up from the kiss.