Hana is portrayed as having been the morality pet for Kaname. This guy was introduced in the Hail of Corn arc as having been friendly and favoring students that he saw potential in, with Ango even going so far as to describe him as more of an older brother than a teacher. But Kaname was quickly shown to be coldhearted, having no care for the students themselves, and seeing them die as part of the Final Test without blinking. At the end of the arc, he's introduced to a newborn Hana and her father, a friend of Kaname's, asks him to please teach Hana some survival skills and help prepare her for the titular project like their previous students. However, Kaname ends up being much gentler and caring towards Hana, teaching her how to survival in wilnderness without the horrifically traumatic ways the previous students had been subjected to.
She was also one to her own father, to an extent. After she was born, her father wanted her to become part of the 7 Seeds Project, so that she would manage to survive and live on into the future. He asked Kaname to teach her because he doesn't want to subject his daughter to the harsh teachings he gave himself as an instructor. However, her father is overall shown to be much gentler when the focus is directly on him, and not told from other people's perspective, like Ango's or even Mark's.
Krista is this to Ymir, 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. All of this goes RIGHT out the window when they learn that she is the Female Titan.
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. In comparison, his mother turns out to be an Inversion. Her influence is repeatedly shown to have brought out the worst in him, and pushed him to lash out at others because he fears disappointing her.
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.
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 the Band of the Hawk including Guts and Casca were almost this for Griffith, but he ultimately sacrifices them. Then after becoming Femto when he returns to the physical world, he visits the memorial for the Band of Hawk and sees Guts and Casca just to prove to himself that he doesn't have a morality pet.
Black Butler: Sebastian has his cat friends (and possibly Ciel himself depending on character interpretationwhether he genuinely cares about Ciel or if he wants nothing more than to eat his soul), Ciel has his household staff and Elizabeth.
This may have something to do with threatening Garcia being her Berserk Button as well, however.
Bleach: 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. Gin may actually be a subversion, since the desire to avenge his only friend is the very reason why he became a bad guy in the first place.
This trope is one of the very, very, VERY few things to be played straight in Bobobo-bo Bo-bobo, where Beauty is this to the title character.
In Brave10, Isanami is one for the anti-hero in Saizo. Were it not for her intercession, Kamanosuke wouldn't have lived to join the Braves.
In A Certain Magical 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).
Raki fills this role for Clare in Claymore. Of course this trope was even more pronounced in the flashback arc when Clare herself helped the legendary Theresa of the Faint Smile reconnect with her own humanity.
Dance with Devils has the main protagonist, Ritsuka Tachibana, becoming this to her school's student council members...who are manipulative devils with negative character traits. Thanks to her influence they show their more softer and caring personalities, effectively transforming from straight-up jerkasses to jerks with hearts of gold.
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.
Delicious in Dungeon has the orc chief Zon's young son, who acts as one for his father. It was him who shamed Zon into sharing the bread they made with Team Thorden rather than letting the party go hungry.
In Detective Conan, the series' resident Femme Fatale and major antagonist Vermouth has two: The main protagonist Shinichi, and his Childhood Friend and Love Interest Ran. After they unknowingly saved her life after she attempted attacking them while she was in disguise, Vermouth becomes very attached to both of them and nicknames them 'Cool Guy' and 'Angel' respectively. Despite being a serious threat in other ways, she is incapable of harming them, even at the cost of her objectives. She later even states that they are the only two treasures she has in the world.
Wormmon to his partner Ken Ichijouji from Digimon Adventure 02, after Ken's repentance. (Unlike the standard depiction of this trope, 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 assignment was to kill the kid, but when the time came, she just couldn't.
Another case like Kari and Gatomon is in Digimon Universe Applimonsters. While Yuujin is the one that usually acts like a loving parent toward Offmon, it goes the other way around once Offmon evolves. Shutmon, with its uncontrolable desire to destroy everything, manages to change himself as a protector instead of destroyer, all because of Yuujins trust in him. Though they both must go through Kick the Morality Pet situation at first.
In the non-canon movie, Dragon Ball Mystical Adventure, Chiaotzu becomes this for Tien Shinhan, who was supposed to kill him to make place for The Crane Hermit and Tao Pai Pai (since Chiaotzu is an emperor in this continuity), but refuses due to the bond they've formed.
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.
Vegeta's family forms this for Vegeta later in the series. Ironically, he is aware of this (and not pleased with it), and has to resort to magical brainwashing to perform a real FaceHeel Turn and become more powerful. Luckily, his love is strong enough that the change doesn't stick. He also seems to get hit with Redemption Equals Death.
Majin Buu shortly after gives up his wrongdoings with some help from none other than Hercule and later Bee. Unfortunately this results in yet another Redemption Equals Death situation when Buu loses against Evil Buu, Thus creating Super Buu. Also like the above examples, Majin Buu eventually comes out okay.
Shinra ends up fulfilling 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; naturally, this is probably the only kind thing that has happened in Izaya's life.
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 crippled 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 wife's 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...
Holland is a major Jerkass 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.
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 the first anime Bradley's pet was his son, Selim. Until he appeared he gave off an ambiguous 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.
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 Jerkass to an outright hero.
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.
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.
Gunslinger Girl: Given that Giuseppe 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 Giuseppe seems. Henrietta, naturally, protests that he's "the kindest man in all of Italy!" The politician wisely lets the matter drop.
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."
Haou Airen. Kurumi Akino became this for Uo Hakuron after saving his life when he was wounded in the streets of Tokyo. Even more so after she learns his Dark and Troubled Past, since she actively aims to become this for him.
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.
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.
Kaguya holds Shirogane in the highest regard, and she contains most of her ill-intended side so that she doesn't alienate him. By Shirogane's example, Kaguya has learned that she cannot possibly continue treating her peers as inferior if she expects to be at his side, and she is forever thankful towards him for forcing her to bring out the very best of herself where there was only a resentful, unfeeling husk before he showed up.
Kaguya is significantly kinder to Ishigami than she is to others once they fall into their Like Brother and Sister dynamic. So much so that she doesn't treat him any differently while Kaguya (Ice) is in control, something that not even Shirogane can lay claim to.
Then there's Mukuro whose HeelFace 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.
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.
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.
Izuna was his brother Madara's entire world. Madara is the man he is today because of Izuna's death.
Believe it or not Itachi is this for his partner Kisame, while Kisame gets his kicks out of violence Itachi prefers to get through without having to fight, most notably during the Search for Tsunade arc.
Speaking of Sakura, she is this to Sasuke. This was constantly Zig-Zagged throughout the series, because his desire for revenge and powers was more important than her feelings. However, it was finally played straight at the end; it was first shown by his genuine apology to her for his selfishness. Later, in Sakura Hiden, Konoha manages to contact Sasuke and request his assistance to help against an enemy who wants to attack Konoha, but he turns them down, as he believes it does not concern him. However, once he finds out that the enemies have captured Sakura, he immediately rushes home.
In Noein, cute little Miho serves as Jerkass 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 Jerkass 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.
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.
In Pokémon, we have Mairin acting this part to social Anti-Hero Alain. Taking this trait away from her would otherwise reduce her to Flat Character status.
Rana serves as one of these for the villainous servant Airi in Queen's Blade. She literally starves herself to death (but she recovers ) due to the kid's influence.
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 HeelFace 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...
Reimei no Arcana: Nakaba is one for both Loki and Caesar. Vernas' younger sister Lemilia is one for Vernas.
Rosario + Vampire: Tsukune is somewhat this to Inner Moka, and tends to keep her from going too far, first seen when he defended Kurumu.
From Shaman King: Hao's androgynous companion, Opacho, who uses toddler-speak and turns into a sheep. D'aww.
Skip Beat! has Kyoko purposefully get hired to be Ren Tsuruga's morality chain during his stint as 'Cain Heel'. Lory was concerned that acting as Cain Heel will lead to old wounds opening up for Ren, so he brings Kyoko in as 'Setsuna Heel', Cain's little sister. Kyoko helps by keeping Ren from slipping too far away and getting him out of stupors — she was the only one who could get through to him, when a car-race stunt almost resulted in an accident, and it was her voice that shook Ren into saving his co-actor and himself, when he got too into his bloodthirsty role and no longer cared about actual injuries. Lory knew that she was the only one for this job. The fact that he was also using this as a form of 'rehabilitating' Kyoko's views on love and playing a bit of matchmaking was just a side-effect.
From Slayers: Not quite to the same extent, but Zelgadis acts far less like a Jerkass 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.
In Spider Riders, Princess Sparkle plays this role to Grasshop, eventually leading him to switch sides.
Also, to an extent, Aqune to Buguese. At the very least, she's the only one whom he'll ever put before his objectives.
Yubaba's son Boh 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 genuinely befriends No-Face.
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.
Downplayed with Ralph in Str.A.In.: Strategic Armored Infantry 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.
In the sequel to Tokyo Ghoul, Haise Sasaki seems to be one to his mentor and surrogate father, Arima. However, it isn't clear whether this is genuine affection or merely a way to manipulate him. Either way, their interactions involve considerable Petting of the Dog and show a more human side to the ever-aloof and mysterious Arima.
In Tomica Hyper Rescue Drive Head Kidou Kyuukyuu Keisatsu, Sasagawa becomes one for Karigari, though doesn't seem to be aware of this herself. Yet his crush on her does lead to some of his more noble behavior and a noticeable decline in revenge plots. Most glaringly in the movie, when Karigari comes up with a way to save the entire world because she's sad.
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.
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.
Alexis is this to Starscream in Transformers Armada, once he switches sides to the Autobots. She's the first person to really develop a friendship with him, and they end up with a very deep bond. (And then he switches back to the Decepticons, although she's still kind of his morality pet.)
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.
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 Yu-Gi-Oh ZEXAL, Hart Tenjo is this to his big brother, Kite. So is Rio Kamishiro to her brother Ryoga (Shark).
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 fulfil 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.'
Yuri!!! on Ice: Yuri Plisetsky is a brash, abrasive and angry Jerk Jock that acts as a huge jerk to the main duo for no apparent reason, though it's hinted that he actually likes them and is confirmed to admire Yuri Katsuki. He does though has people with whom he acts like a sweet guy, first his grandfather Nikolai and later, his Only Friend Otabek.