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.
Istvan Bathory is this for Vlad Tepes in Count and Countess, although Vlad barely tolerates his presence and only acts kindly to him because he is Elizabeth's great-grandfather.
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.
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.)
It is mentioned that the most deplorable criminals in the Tanty (an Ankh-Morpork prison)are given a songbird as a companion, and every one of the men cares for his songbird.
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 Dragon Blood, it is Ward himself who becomes a morality pet, to Oreg, who is not as dead as originally assumed. As a very decent character, Ward makes for a good morality pet; his allies, while none of them is evil, are somewhat less concerned with justice than he is, and more concerned with his safety.
In Dragon Bones Oreg acts as one of those. There is a brief moment where Ward considers to help some nobles re-capture a slave that fled to his lands. Then he remembers Oreg's suffering, and decides that slavery is wrong, period, and he will face all the difficulties this decision entails. Having another "human" being completely at his mercy (Oreg is magically bound to him and he can't change that) helps Ward to be a better man than he might otherwise be — he could not bear the thought that his slave fears him.
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.
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.
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.
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.
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.
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 an interesting twist, Lucius, Narcissa and Draco Malfoy all seem to play this role for each other 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 toonote assuming she was ever allowed to act on her free will while serving Voldemort...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.
Mags to Finnick in The Hunger Games, as it also works on our heroine. Katniss immediately wanted to ally with Mags because she volunteered to the take the place of another woman, despite being elderly and having absolutely no chance of fighting off any of the younger, stronger tributes (and thus, very little value as an ally). Meanwhile, she views handsome, physically fit Finnick as one of her most dangerous enemies, and one of the most ruthless killers. Now, throw them together in the arena and Finnick tells her point-blank that he can't leave Mags and proceeds to carry her around on his back, because she's his mentor and one of the few people who genuinely likes him.
Prim also serves to humanize Katniss.
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 P.G. Wodehouse's Jeeves and Wooster 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.
Journey to Chaos: Eric argues with Grey Dengel about whether or not Asuna was one of these to the real Dengel. Eric's argument is that being sweet on one person doesn't mean he wasn't an overall awful person. Grey Dengel's counterargument is that, by the same token, it means he wasn't completely heartless and evil.
Benji, the Supporting Protagonist, is the key to redeeming Zarracka because he is the first person to be nice to her in years.
Invoked by Finn. When he wants to prove that he's not evil he points to his loving wife. "She wouldn't have married me if I were evil."
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 the fifth book of the Malazan Book of the Fallen, the slave Udinaas serves as this to Rhulad, the Emperor of a Thousand Deaths. He is fully aware of it and even lampshades it after a battle where many of the enemy soldiers had been slaughtered by chaos magic: Udinaas comments on how Rhulad had been furious about the outcome, but only because he had wanted to fight and die — the other sentiments, like anger about the senseless slaughter, had been borrowed from Udinaas. He later mentions how the role he served should have been occupied by someone from Rhulad's family, but they had all been too busy feeling sorry for themselves.
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.
In [[The Night Angel Trilogy, Ulyssandra acts as a bizarre example of this for Vi, in that the only reason it counts is because Vi didn't decide to kill her immediately.
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 Sandor Clegane at different points.
Davos is Stannis's Morality Pet. Their relationship is one of the only really likable things about Stannis.
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.
In the Time Scout series, Marcus is this for Skeeter, before his Heel–Face Turn. Afterward, he's devoted to the downtimers in general, but Marcus and his family are special.
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.
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.