In most single-player video- or computer-game RPGs, NPC party members aren't so much Morality Pet as they are Moral Compasses — 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.
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 badass 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.
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 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.
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.
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.
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 pooping all over one's followers?
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.
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.
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.
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. At one point Isabela, a pirate captain, cheats her at cards, notices that one of the things Merrill bet was a clan heirloom, and immediately gives it back.
It doesn't even have to be a Romance Sidequest. Some characters, like Merrill (above), Areie, Deekin, Blizz, Nadia Grell (for a female Consular or a male not romancing her), 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 because Little Brother Is Watching. More than a few gamers have immediately noped out of the Dark Side ending of Knights of the Old Republic because you have to kill Mission in it.
The player character can act as this for Solas in Dragon Age: Inquisition. He admits to a befriended or romanced Inquisitor that, prior to meeting the them, he didn't even see the inhabitants of modern Thedas as people, because they were so different from the elves of the pre-Veil world. Getting to know the Inquisitor and the other members of the inner circle changed his mind, and made him see that there were things in this world worth saving. It isn't quite enough to sway him from his path, but he does show a willingness to aside his plans if the Inquisitor can find some sort of alternative.
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 first game, Judau Ashta becomes one of these for Paptimus Scirocco. Not only does he convince Zeta'sBig Bad to pull a HeelFace Turn in the story they share with Judau's Love Interest Roux Louka, but in the Heero/Jerid/Master Asia story, Scirocco immediately reverts to his treacherous his nature in the final mission, but only after Judau is killed in battle with one of your three Villain Protagonist characters.
Sheogorath, the Daedric Prince of Madness, has one in Haskill, his Only Sane Man/Servile Snarker chamberlain. Given that Haskill's origins are unknown, and Haskill himself claims to have been in the service of Sheogorath "since the beginning", this has led to the theory that Haskill is an external part of Sheogorath, similar to what Barbas is to Clavicus Vile. Haskill reigns in Sheogorath's madnessjust enough to keep him and the Shivering Isles functional.
In Skyrim, Morthal's Court Mage is a sketchy, highly unpopular wizard by the name of Falion. He's abrasive, sells Black Soul Gems, and sneaks off at night to perform uncouth rituals in the nearby swamp, most likely related to necromancy. The entire town (minus his own sister and the already-crazy Jarl) knows he's up to something. However, he has also taken in a young orphan girl and is motivated by a desire to keep her safe. In turn, she adores him.
In Ensemble Stars!, Souma served as this for Akatsuki, and in particular for Keito. The unit was founded solely to carry out Eichi's will in sabotaging the five oddballs, but because Souma truly believed that Akatsuki could be great and that they could make people happy as idols, Akatsuki did eventually become that. In the present year, Keito's affection for Souma is his primary source of Pet the Dog moments showing he isn't always strict and cold. However, Souma didn't have it as easy as some other morality pets - Keito dragged him into Akatsuki's less than wholesome work as well, and when Souma became very upset and actively argued against their sabotage of Kanata, who he genuinely really cared about, Keito yelled back some awful things at him and expelled him (temporarily) from the unit. Eventually Souma's vision wins out, but for a while there, it wasn't clear it would end that way.
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.
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.
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. It doesn't stick, but her influence changes the ending.
Kratos gains another such pet in the PS4 reboot/sequel in the form of his son, Atreus whom Kratos mentors to survive.
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".
Halo 5: Guardians, played with in regards to how Cortana treat Master Chief and Blue Team. The former holds Blue Team in high regard and protects them from dangers but this behavior does nothing for their mortality as Master Chief accurately identified that this was a psychological tactic to keep him and Blue Team on Cortana's side.
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.
In the H-GameIdols Galore, the protagonist would otherwise be a Jerkass 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.
Arguably, Melissa Woodward, in Hotel Dusk: 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 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.
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 literal Morality Pet, his parrot. If you shoot her, 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.
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.
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.
In Odin Sphere, Gwendolyn serves as one for Oswald. His treatment of her is in large part what spurs him into being a more heroic character.
Invoked in Persona 5; when Funny Animal Morgana moves in with you, Sojiro allows it because he figures having a pet to take care of will keep you on the straight and narrow. (Also, he thinks it's an adorable little kitty cat.)
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.
Subverted by Ghetsis of Pokémon Black and White, whose Hydreigon has Frustration, a move that can only be used if the Pokemon hates it's user, implying that he's actively abusing it to maximize it's power.
Lysandre in Pokémon X and Y might not have a Pokémon what evolves through happiness, but he has a Mega Gyarados in his team. Story-wise, Mega Evolution can only be achieved if the trainer and the Pokémon have deep enough of a bond with each other. Shauna even says that the two of them must work close together for them to share such power.
Gladion in Pokémon Sun and Moon has three of these. His Zubat and Type: Null evolve into a Crobat and Silvally by the end of the game, and he also obtains a Lucario at some point between the point where you storm Aether Paradise with him and when you meet him again at the Pokémon League.
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.
From the [PROTOTYPE] games:
In a city of twelve million people, there's only one person Alex Mercer from [PROTOTYPE] 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.
In Prince of Persia: The Two Thrones, the eponymous Prince is initially just concerned about reaching and killing the Big Bad who has invaded his kingdom, a mindset encouraged by theDarkPrince. However, when he teams up with his long-lost love Farah, she reminds him — sternly if need be — that he has the responsibility to save his people, even if it hinders his efforts to locate the Big Bad. He also begins to wonder through her reprimands if listening to the Dark Prince is doing any good. At one point, she storms off to rescue people when the Prince wants to press forward. He almost instantly runs after her, concerned for her safety. Kaileena lampshades this by saying that now the Prince was caring for someone else than himself, even if just for one person. When the Prince is trapped in a mental battle with the Dark Prince, he's able to get rid of his dark alter-ego only when a manifestation of Farah advises him to just abandon the fight and follow her to the light.
A non-villanous example is explored in the Ratchet & Clank Future saga, where Clank is this for Ratchet. Ratchet is a Lombax, whose hat includes an outlook summed up as "Reward over Consequence". While Clank is normally the one to offer a rational outlook and approach in all the games, it's brought out fully when Ratchet is separated from Clank and spends time with fellow Lombax Alister Azimuth. As soon as the two reunite, Ratchet follows Clank's more rational advice, which puts him at odds with Azimuth.
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.
Leon is this for Ada Wong in the Resident Evil series. It's very strongly implied (and confirmed in the non-canon S.D. Perry novelization) that her relationship with him is what pushed her out of being a heartless secret operative to an Anti-Villain/Anti-Hero with heroic tendencies.
Amy Rose from Sonic the Hedgehog towards E-102 Gamma, Silver, and to a lesser extent Shadow (heavy influence on his HeelFace Turn due to reminding him of Maria).
Raphael from the Soul 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. 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.
Luca Blight from Suikoden II may be an Ax-CrazyOmnicidal Maniac, but he really does care for his sister, Jillia. 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.
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. That said, Medic has several moments of real humanity with the doves. He gently pets one of the doves before leaving for battle in the "Mann vs. Machine" video. In the comics, Medic is horrified when Classic Heavy kills Archimedes. He spends the rest of the scene feverishly reviving the bird. Classic Heavy's insults and cruelty toward his pet drive Medic to betray the Classic Team and rejoin Team Fortress.
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.
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.
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.
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.
Undertale provides an amazing example of this for you, the player. The game employs a Slowly Slipping Into Evil mechanic where you're free to kill as many monsters as you like, and won't realize how bad you actually are it's too late. However, when you encounter Papyrus, the second boss, he is just so damned nice, lovable, good-natured, and innocent, that he's the one who pushes many people to abandon what would be a Genocide Run and instead begin sparing monsters.
In Super Robot Wars X, Wataru becomes one for Ange. Ange says more than once that she just can't say no to him.In fact, Ange's interactions with Wataru play a role in making her character more benevolent and reasonable. So much that when she finds Sylvia walking on her own, she offers her a hand to restore order to Misurugi and establish Cafe Ange in her old home.