Zen from Akagami no Shirayukihime manages to inspire loyalty in all three of his retainers, though most strongly in his first retainer the knight Mitsuhide. Kiki knows she will have to leave his side as one of his knights eventually due to being next in line as head of an important noble family. Obi originally intended to just stick around for a short time since he had a personal policy of never forming strong ties with people, considering them fatal for people in his line of work. Eventually, when he realizes he's stayed too long as he's completely devoted to Zen and Shirayuki, he says he stayed in a troublesome place but no longer regrets it. The men in the fortresses he manages are also very loyal to him.
Yaiba has Brau, who is eternally grateful to the prince for accepting him despite his scarred face.
Grenada is protected by Amarello, who considers protecting the queen the thing that gave her life purpose.
In Beelzebub, the King's Crest is granted to those whose allegiance is sworn to a demon lord, which gives them a massive power boost. As of Chapter 217, it has been revealed that all four members of the Tohoshinki bear Beel's King's Crest, all but stating that in some way, all of them are loyal to Oga (even if they never admit it).
When questioned by Fuji on this, Himekawa laughs and states that Fuji's inability to comprehend the loyalty many of them have to Oga will be his downfall. Considering that Himekawa willingly allowed himself to be turned to stone in order to give Oga time to escape and stall Fuji, he certainly embodies the trope as well.
Black Butler: All of the Phantomhive staff are eternally devoted to Ciel, for various reasons — Tanaka because he'd been with the family since before Ciel was born, Sebastian because of their contract (though it's hinted he genuinely likes Ciel, in a Vitriolic Best Buds sort of way), and the rest because Ciel saved them from horrible lives and gave them a new purpose. Though that one's also a Tear Jerker when we find out that Ciel and Sebastian manipulated them for the sake of their loyalty.
Ciel himself is undyingly loyal to Queen Victoria. Whenever she has a mission for him, he drops everything on the spot in order to carry it out for her, even if he doesn't particularly like the assignment.
Tsukishima has an ability to deliberately invoke this trope to anyone he cuts. His power allows him to manipulate existing bonds so that a false bond between himself and the victim can be inserted and supersede the bond it's designed to override. He also has an Undying Loyalty of his own. Ginjou was everything to him, having found him and looked after him from the time Tsukishima had been a small child. Tsukishima admits that Ginjou had taught him everything...except how to live without him.
When Byakuya is brainwashed, his loyalty to Ichigo is so extreme that it gives him the power to kill his enemy — not because the brainwashing failed, but because he would kill even someone he loved if they were an enemy of Ichigo's. Tsukishima learned the hard way that he had manipulated the wrong bond when he discovered that the emotional bond that most motivated Byakuya wasn't the bond between mentor and student, it was the bond between Byakuya and Ichigo.
Komamura's Undying Loyalty to Yamamoto stems from Yamamoto having once saved his life and always treating him well despite his appearance. When Yamamoto enters battle against the Stern Ritters, both he and the Seventh Division are on the verge of catastrophic defeat. His response to Yamamoto's resolve to fight is to rally both himself and the entire Seventh Division, much to the disbelief of Bambietta, who had thought she'd won and all but killed the squad.
Katagiri was raised to be Ryuuken's personal servant and bodyguard. She will protect him no matter what the cost and her loyalty is to Ryuuken personally, over and above her loyalty to the Ishida family. So, when he believes he's failed his entire clan and tries to slink off forever into the Grey Rain of Depression, Katagiri tells him that if he leaves, she'll abandon everything to go with him. Her speech brings him back to his senses.
Taken Up to Eleven in this case. Even after being zapped by Auswählen, Haschwalth doesn't consider Yhwach's act as a betrayal, but rather an honor and point of pride that Yhwach sees him as an asset worth absorbing. Damn...
Blood+: Chevaliers are biologically wired to be absolutely dutiful and devoted to the Queen who turned them though it is possible to overcome this, as shown by Diva's Chevalier, Solomon, when he falls in love with the rival Queen, Saya.
Haji in particular is characterized almost entirely by his unswerving loyalty to Saya. Haji's devotion makes him reluctant to speak out when Saya makes decisions that might be bad for her; he even promises to kill her once she succeeds in killing Diva, and it takes being confronted and berated by Saya's brother Kai before Haji will admit that he would much rather not go through with it.
In Brave10, many of the Braves end up feeling this towards Yukimura and the lands of Ueda, even Saizo, to his own surprise. The best examples of this trope among the cast are Sasuke, Rokuro, and (in the sequel) Nanakuma, all of whom never waver and will pursue loyalty past the point of reason.
In The Bride of Adarshan, Alec is fiercely loyal to his brother, the king of his country, to the point of having few motivations outside of serving him. The idea that he, as the prince, is in a position where he could usurp his brother never even occurs to him, making it a surprise to him when he discovers that others fear this.
Habaek in The Bride of the Water God gave his first bride Nakbin the right to order his death, tried to follow her in death when she died, endured being separated from his current love Soah just to be with the resurrected Nakbin and when she died again, he honored her command not to look at someone with the same loving eyes he used to look at her even to Soah when he specifically told her that he will look at Soah only with his one eye (metaphorically though since his damaged eye is completely healed as a god.)
Dune in Casshern Sins. He was Luna's bodyguard and was extremely royal to her. When he reunited with her again in the ruined world, he keeps his loyalty until the very end, despite Luna becoming a jerk.
In Claymore, Jean would go to literally suicidal lengths for Clare, as a result of I Owe You My Life sentiment. Also, Miria tends to inspire this in most people she meets, especially fellow Claymores.
To top it off, Jeremiah's loyalty to Lelouch comes from his loyalty to Lelouch's mother Marianne. Now that's truly undying. He even stuck with Lelouch all the way through the Zero Requiem; he's so loyal that if Lelouch wants to pull a Heroic SacrificeZero-Approval GambitThanatos Gambit like that, he will support him.
Gilbert Guilford is the same towards his liege Cornelia. He's just much quieter about it. He's obvious enough that Lelouch can take advantage of it though.
Kallen Kōzuki/Stadtfeld also holds this toward Lelouch, such that the latter has to seriously go out of his way to have her stand against him during Zero Requiem, by breaking her heart and forcing her to choose to stand by her ideals and fight against him instead of staying by his side. Kallen confesses in her character poem that if Lelouch had given any indication that he loved her, even if it had been a lie, she would have followed him to hell. She then goes on to say that although she knows Lelouch had purposely kept quiet for her sake because he had known that she would have chosen to stay with him till the end, she did not fall in love with him because of this kindness (Implying that she wishes he would have been honest with her, and let her make her own decisions, instead of keeping her in the dark to protect her).
Meido Sayoko Shinozaki is totally devoted to the Lamperouge siblings, to the point where she's the only Black Knight who remains loyal to Lelouch for the entire show.
Despite being the most reluctant of his followers at first, Tamaki became this to Zero. It took nothing less than the warped information that he's responsible for the SAZ Massacre for Tamaki to turn on him and even then he still believes in him until he goes Demon Emperor on the world.
Cute and PsychoRolo became this to Lelouch (noticing a pattern?) thanks to him being the first person to be nice to him and is so devoted he kills himself rescuing Lelouch even after he chased him off with the reveal he never cared for Rolo.
In Demon Slayer: Kimetsu no Yaiba, that's the shining quality that makes Zenitsu stand out beyond his apparent cowardly and whiny behavior; Zenitsu will bury all his flaws within when it comes to giving his whole self for people who were honestly kind to him and believed he could reach greater success in life. Zenitsu is at first scared of fighting demons as he fears he could die at any moment, but Zenitsus master trained him with so much care that he will never abandon the sword; Tanjiro is such a good natured person that Zenitsu pretty much decided quick enough that Tanjiro is someone who he should unquestionably trust and put his life on the line by facing all sorts of mortal battles with.
Hisoka Kurosaki from Descendants of Darkness, to his partner Tsuzuki Asato. When the latter is going through the Despair Event Horizon, Hisoka assures him that he will always be there for him even if everyone else isn't. Later, when Tsuzuki tries to commit suicide, Hisoka, having given up on trying to reason with him, holds onto him so that he'll die right by his side.
Puar for Yamcha. She's known him for ages, having been his partner in crime when the series first began.
In Dragon Ball Super Ginyu to Frieza. He waited almost two decades to find a proper body so he can return to Frieza's service once he's revived.
Kale to Caulifla. She has always been by her side since the day Caulifla rescued her when she was lost by a roadside and is eternally grateful.
Sorbet to Frieza. He goes through a lot of trouble to recieve the Dragon Balls and use them to resurrect Frieza to bring the Planet Trade Organization back into power. Sorbet will always stay by Frieza's side till the end, even in death.
Berryblue to Frieza. Like Ginyu and Sorbet, she is loyal to Frieza till the end since she was his childhood nanny and second-in-command.
The Mustang unit to their commanding officer (and also to one another). Particularly true of Hawkeye, who at one point, refuses Mustang's direct order to abandon him and save herself if his attempt to expose the Government Conspiracy fails. It cuts both ways: Mustang will do anything to protect his subordinates (again, particularly Hawkeye, to the point of jeopardising a secret mission to save her).
Fu and Lan Fan to Ling
And of course, the Elric brothers to one another.
The Briggs Brigade to Major-General Armstrong; Pride to Father.
The Homunculi (sans Greed) to the Big Bad of the series (Father in the manga and Brotherhood anime, Dante in the 2003 anime). Within the Homunculi, Gluttony has this for Lust in both series, and also in both series, her death affects him very badly.
In The Conqueror Of Shamballa, Gluttony completely ignores Alphonse and goes exclusively after Wrath - Lust's killer. Keep in mind that, after the events of the 2003 anime, Gluttony is supposed to be completely mindless.
Gintama have several exemples, often notable as it come from characters who are otherwise free-spirited and independent:
The most frequently seen exemple is perhaps, Hijikata and Okita's loyalty toward Kondo. In Okita's case, it is often shown that he is closer to being a assassin than a hero, he is on the side of the law mostly because this is where Kondo led them. And Hijikata made it clear in a early episode that his loyalty does not go to the government or the law, but to Kondo, as he is the one who gave them a place to stay and a way to keep their katana after the Shogunate made them illegal. This mindset extend to most of the core members of the Shinsengumi due to Kondo being A Father to His Men.
Both Gintoki, Takasugi and Katsura where extremely devoted to their mentor and father figure, Shoyo Yoshida. This was exploited when Oboro gave Gintoki a Sadistic Choice between killing Shoyo or watching Takasugi and Katsura die. He knew that both of them would've rather died than be the cause of Shoyo's death, and he would have himself prefered to save his teacher. But out of loyalty for his master, he chooses the first option, because he knew that was the choice Shoyo, willing to die so that his students be spared, wanted him to make.
Matako, Bansai and Henpeita of the kiheitai, are willing to follow Takasugi through anything and never let him down even in time of hardship. After the tendoshu and the harusame turned on them and try to have them eliminated while Takasugi is on the verge of death, they still stay on his side and do their best to save his life.
Mutsu, for how much she chastise and reprimand Sakamoto, is extremely loyal to him and never let him down. Same go for the rest of the Kaihentai, due to Sakamoto also being A Father to His Men.
Patrick Colasour embodies this for his superior officer, Kati Mannequin in Gundam 00. The reason is love. The undying loyalty is his cockroach like tendency to survive any dangerous No One Could Survive That! situation, especially on her orders. Eventually, it pays off big time with him winning her hand in marriage.
Gungrave. Brandon's personal code of loyalty puts him under much torment when he has to choose between the best friend Harry and the father figure Big Daddy. Brandon's inability to choose ends up being one of the main causes of Brandon's downfall, which in turn leads to suffering of the people closest to Brandon. In the finale, Brandon admits that even though he is supposed to enforce the iron law of the syndicate to never betray, he just cannot shoot his best friend Harry.
Alucard of Hellsing has this for Integra Hellsing. On the Catholic side, the priests and nuns of Iscariot Section XIII are eternally loyal to Badass Preacher Alexander Anderson, most of whom were raised in his orphanage and view Anderson as a surrogate father.
Byakuya from Inuyasha is the only one of Naraku's detachments shown to be completely loyal to him, and he's also the only character in the entire series who doesn't completely despise Naraku and does not betray him.
Robert E.O. Speedwagon is extremely loyal to Jonathan Joestar via Defeat Means Friendship, combined with his selfless rescue of him from a burning building/Vampire attack. This loyalty would extend to the entire Joestar family, even more than half a century later.
On the villain side, we have Vanilla Ice. How loyal is he? He cuts off his own head when Dio says he needs blood. Dio actually turns Ice into a vampire in thanks for this. When he fights Iggy and Polneraff, he delivers a No-Holds-Barred Beatdown to Iggy just for making him destroy an effigy of Dio.
Enyaba is also extraordinarily loyal to Dio, to the point that she refuses to reveal the secret of his Stand even as she lays dying, having been killed in a really sadistic way on his orders.
Literally the case of Koinosuke Odago in Jubei-chan, became undead to follow his master's directive through sheer force of will for 300 years.
The defining trait of pretty much all Clansmen in K. This relationship between Kings and their Clans is a central theme of the series.
Kuroh Yatogami, who enters the story on a mission from his late master, and finding a new King to give his loyalty to. His faith in his King gives him strength. "I wield my sword only to protect him. My sword will open up the path and lead my to my King!"
Misaki Yata, third-in-command of the Red Clan, idolizes the Red King — both Mikoto and his successor.
And most notably, Saruhiko Fushimi, former Red Clansman and third-in-command of the Blue Clan. He felt none of this towards the Red King Mikoto, and scoffed at HOMRA's awe and loyalty towards him. When he left and joined Scepter 4, he didn't want to let himself feel this way towards the Blue King. It wasn't until Misaki, his friend-turned-enemy-turned-friend again, and the only HOMRA member not to realize he wouldn't stay with them, realizes it and tells him, "You're not a traitor — if you would go this far for the Blue King, that means he was your King all along!", that he realizes he feels this.
Katekyō Hitman Reborn!: Gokudera towards Tsuna is this trope, completely and totally. Heck, his whole character song seems to be a tribute to his undying loyalty towards Tsuna.
Arguably, Squalo qualifies as this for Xanxus. Not only did he wait 8 years for Xanxus' return, but he grew out his hair and puts up with crap that would make a Bastard Boyfriend seem like a pleasant alternative.
The Elite Four are all unshakably loyal to Satsuki, each one ready to give his or her life for her at a moment's notice (especially Gamagoori).
Senketsu is completely devoted to protecting Ryuko (earning him the Fan Nickname "Broketsu").
Nui is undoubtedly loyal to Ragyo, by the finale she proves this by killing herself when Ragyo ordered her to.
Lupin III: Inspector Zenigata has the ability (Depending on the Writer) to inspire this in cops from all over the world. His subordinates in the Japanese police have ignored orders from the Interpol to follow Zenigata, and in the special Voyage To Danger the Italian police, after trapping Lupin in an inescapable trap (Lupin himself was desperate), packed and let him go when they learned that Zenigata had been pulled from the case because if they got him in that occasion it would have meant that Zenigata hadn't been the one who arrested him.
The Familiars and Wolkenritter. If it's for the sake of their master, the rightness or wrongness of an act doesn't matter, and if it will benefit or save their master, they will gladly give up their lives, powers, and/or freedom.
In the Wolkenritter's case, this overlaps with My Master, Right or Wrong, as it was implied that some of the masters of the Book of Darkness were evil. This makes itself prominent in A's, where Signum stated they would gladly throw their honor away if it means saving their mistress as well as during StrikerS where Vita stated that she did not have any plans of outliving Hayate. It's also literally undying, since the first thing they said after being resurrected was a Badass Creed about their loyalty to Hayate.
Additionally for the Wolkenritter, they are loyal enough to Hayate disobey her for the hope of saving her life, which is what kicks off the plot of A's.
Fate Testarossa as well, initially toward Precia, then Nanoha.
Several of the Numbers have this towards each other, most of them going so far as to declare that where one goes, they'll follow. Case in point: Sette to Tre.
The various Intelligent Devices in the series all show unquestionable loyalty to their weilders, best demonstrated in A's when Raising Heart and Bardiche demand to be upgraded with the Belkan Cartridge System so they could better protect Nanoha and Fate.
Sarah Zabiarov from Mobile Suit Zeta Gundam is devoted to her superior, Scirocco, something he actually reciprocates somewhat. She's so loyal in fact that not even death stops her, her soul attempting to stop Kamille's Newtype-infused waverider attack on Scirocco himself. It takes Katz for her to finally step aside.
Psycho for Hire Roberto from Monster is unfailingly loyal to his master Johan Liebert. Though given that this is Johan Liebert, a man often compared to Hitler and the Devil himself, it's unlikely that he feels the same.
Haku from Naruto to Zabuza, who rescued him while he was an orphan struggling to survive in the city.
Kimimaro to Orochimaru even after being resurrected.
Konan to Nagato.
Also Kisame Hoshigaki to Tobi.
Even after death, Uchiha Itachi is still loyal to Konoha.
Yashamaru to Hidden Sand Village. As a shinobi, he served as a member of Suna's black ops and as the right hand of the Fourth Kazekage, Gaara's father despite hating him for sealing Shukaku into his pregnant older sister and Gaara's mother. It was because of this loyalty that Yashamaru willingly went through the assassination attempt to test Gaara despite legitimately caring for him and objecting to them.
Himari from Omamori Himari combines this with a Bodyguard Crush towards protagonist Yuuto. Her willingness to die for him borders on being a Death Seeker (partially due to her also being an Atoner), and in the finale of the manga her loyalty is literally undying, allowing her to come back from the afterlife.
Each of the Straw Hats are fiercely loyal to their captain, Luffy, ready to risk their lives for him as soon as he gives the word. At one point, when Bartholomew Kuma wanted to kill Luffy, both Zoro and Sanji, without a moment of hesitation, offered their lives instead. This loyalty goes both ways. Luffy is just as willing to die for his crew and, ultimately, when he was at his very lowest, they were the only reason he bothered to keep on living.
Donquixote Doflamingo's crew have absolute loyalty and faith in Doflamingo, even to the point of death. Doflamingo himself seems to care for his crew (as long as they don't betray him), but that does not stop him from letting them kill themselves for him.
Bellamy; even back when he was little more than a thug with a devil fruit, the one redeeming quality he's always had is his loyalty. Unfortunately, his loyalty is wasted on Doflamingo, a man completely undeserving of it, who goes out of his way to spit on that loyalty whenever he can. Even as it becomes clearer to Bellamy that he admired the wrong guy (which he later fully admits to Luffy), his principles and his pride refuse to allow him to turn his back on the idol he has been devoted to for so long.
Also, Daz Bonez towards Crocodile. It's probably why out of all of Baroque Works, it was him Crocodile liked the most and why Crocodile made it a point to free him during the escape from Impel Down.
Trafalgar Law definitely has this for Corazon, a.k.a. Donquixote Rocinante. He outright states that Corazon is his most cherished person, that he only decided to continue living to fulfill Corazon's wishes, and that he would gladly destroy the entire world as long as it was for Corazon's sake. It is to the point that he dedicates half his life to murdering Doflamingo in order to avenge him, and it is this Roaring Rampage of Revenge that culminates in the Dressrosa Arc.
During the Dressrosa arc, several characters banded together to fight alongside Luffy against the Donquixote Family after having come to admire Luffy in the gladiator's tournament many of them participated in and after Usopp saved them from being transformed into Living Toys and made slaves for Doflamingo. At the arc's conclusion, all but a few of them swear formal fealty to Luffy. This is done over Luffy's own objection, as Luffy doesn't want to be some bigshot leader nor be responsible for tying them down. They simply reply that if being free to do as you please is the most important thing to Luffy, then they'll just do what they want and make their oath.
In One-Punch Man, no matter the choices Saitama makes or how he acts, Genos will always be loyal to him, have his back when he needs it, and won't be afraid to stand up for him. After Saitama defeats the Deep Sea King and acts as a jerk to ensure that the other heroes get recognition, Genos has this inner monologue.
Genos: Master Saitama. Are you really okay with this? But if this is the path you choose to take, master, I will accept your choice without a word. However, once you've turned the masses against you will you even be able to work as a hero anymore? I can't help but worry about what's to yet to come. If Master Saitama should ever find himself completely cornered, if it should ever come to that, I will.
The Floor Guardians toward Momonga. It makes sense, considering they were originally NPCs created by Momonga and his fellow guild members, having their unwavering loyalty to their creators (or "Supreme Beings," as they so lovingly refer to them) hardcoded into their personalities. Literally the only way to force one of them to turn against Momonga is to use a brainwashing World Class item on them, an item so powerful that a person could literally rewrite the original game's rules (well, ask the devs to do so) if they possessed one.
However, it should be noted that each of the Floor Guardians is infinitely more loyal to the guild member who created them than Momonga himself, and would instantly turn on him if their creator requested it. Momonga doesn't mind, as he sees it as proof of how much the NPCs care for his friends and their legacies just as much as he does. The sole exception is Albedo, due to Momonga jokingly rewriting her code to be deeply in love with him moments before being transported to the New World, and who is currently putting together a team of Guardians to hunt down and kill the other Supreme Beings except her own creator who they see as having betrayed them because they'd stopped playing the game, mostly due to real life getting in the way, and were never aware that their creations would become sentient, but Albedo's too far gone to understand that.
It runs both ways: Momonga's offline life was so crappy that the game was his only joy, spending large amounts of his income on it and managing the guild, so his guildmates' NPCs are the closest things he has to being with his friends again. On hearing Evileye nearly killed Entoma, he very nearly killed Evileye on the spot if she hadn't been necessary for the plan as a witness.
Their loyalty is such that Momonga finds it near impossible to get constructive criticism from them, because they're ready to off themselves and each other at the slightest hint of disrespect towards their master, even when said master is asking for them to have different opinions.
Demiurge is a scheming mastermind who puts together complex plans to take over the world... all because he misinterpreted an offhand comment by Momonga about how it might be interesting to do so as an official order from the Supreme Being. Despite everything about him screaming "eventual traitor", he also genuinely views Momonga as his superior as an Evil Genius (where he expected to bring the Empire to heel only after long attrition and horrendous casualties on both sides, Momonga convinces the Emperor to submit entirely by accident in a few hours just by showing up), and considers every time Momonga congratulates him on his plans working to be praise from Caesar.
Puella Magi Madoka Magica's Homura Akemi has a strong devotion to Madoka. She repeated the same month over and over again so that Madoka would be safe. Word of Godconfirms this, stating that she was nearing the 100th try by the time the series ended. In the third movie, their descent into insanity is marked by a startling willingness to brainwash this same person and trap her in a Gilded Cage.
In Ronin Warriors, the true virtue the Ancient One imbued in Anubis' armor turns out to be Loyalty. After his HeelFace Turn, Anubis shows why the armor chose him, remaining loyal to the Ronin Warriors and the Ancient One's memory to the point of sacrificing himself to save the Warriors and Lady Kyra, the last member of the Ancient One's clan.
Three of Shishio Makoto's Juppongatana in Rurouni Kenshin display undying loyalty to him. Yumi uses herself as a distraction so that Shishio can stab Kenshin through her. Kamatari had to be deceived so that he did not follow Shishio in death. These two were in love with Shishio. The last, Houji, thoroughly subverts Evil Chancellor. He is evil and he's Shishio's majordomo but he is utterly devoted to his leader. Enough to take the blame for an unpopular plan and risk the ire of the other Juppongatana. Enough to follow Shishio into hell. Even Shishio himself acknowledges Houji's loyalty.
In Saike Matashitemo, Hizu was only fighting on the good side due to his loyalty to Saike. At one point, he died just to get Saike out of amnesia, expecting Saike to not even return for him and at another, he was about the force someone to kill himself to save Saike.
The Sailor Senshi have this towards Usagi and Mamoru as their future Queen and King in the manga. In the original anime the Inners have this towards Usagi, but Haruka and Michiru take an entire season to come around.
Jadeite from the first season is a villainous example. While the other three Generals are relatively obedient to Queen Beryl, Jadeite is the only one who's never questioned her, mouthed off, or given her some sort of cryptic answer when she demands an explanation of his plans. Even when his last failure ensures nothing but punishment for him, he returns to her without question.
Zoisite has this for his mentor and lover Kunzite, even stating at one point that he would throw away his life for him.
Sailor Neptune has this for her partner and lover, Sailor Uranus. Given the choice between not risking dooming the entire world and not risking Haruka's safety, she happily chooses to protect Haruka at the potential expense of the world, even surprising the villain forcing her to make such a choice. Though it isn't clear whether Michiru knew the villain was bluffing.
Sailor Neptune: Maybe you misunderstood...A world without Haruka isn't a world worth saving.
Loyalty is a big deal in the Sengoku Basara anime and games, given the subject matter, but Kojuuro and the rest of the Date clan really stand out in their loyalty to Date Masamune. In the anime's second season, Masamune's followers pile onto him bodily to protect him from Toyotomi Hideyoshi; a flashback in the following episode shows Kojuuro preparing to commit seppuku because he blamed himself for Masamune's arm being wounded in battle.
Yukimura's loyalty to his master Shingen is completely unfathomable. He can't even imagine a world where the latter doesn't exist for him to serve.
That Time I Got Reincarnated as a Slime: All of the citizens of Tempest are fanatically loyal to Rimiru. Part of this is because he saved all of them from various disasters and drastically improved their living conditions, part of it is that he named every single one of them individually (which creates a bond of power between them), and part of it is simply because he's the strongest thing around; monsters greatly respect strength.
After Kotetsu wins his trust in the first half of Tiger & Bunny, Barnaby develops a fierce loyalty towards him to the point that staying by his side becomes his only reason for remaining a hero.
Occurs in multiple places in Tokyo Ghoul and its sequel, :RE.
The staff of Anteiku to Yoshimura. This results in two different outcomes, with Yomo given the responsibility of getting the younger members to safety while Koma and Irimi stay with Yoshimura during his Last Stand.
Banjou may be weak, but his trio of gas-masked followers are utterly loyal to him. When he swears loyalty to Kaneki, they follow suit without hesitation.
In :Re, the employees of the Tsukiyama Group are completely devoted to their master. This is hinted to be a result of the kindness shown to them over the years, and the business empire allowing an untold number of Ghouls to live stable, relatively normal lives in human society. When it all comes crashing down, Shuu is surprised at the massive number of Ghouls that show up to protect him out of loyalty to his father.
In a surprising twist, Yamori is revealed posthumously to have inspired incredible loyalty among his followers. His Number Two, Naki, likewise inspires incredible loyalty after taking over the gang.
Harry Ord from ∀ Gundam toward Queen Diana. He develops this toward Kihel as well (during her time spent pretending to be Diana), but his feelings toward her are tinged with a romantic interest, while his loyalty to Diana is completely platonic.
Despite the fact that their boss Dilandau treats them horrendously the Dragon Slayers in The Vision of Escaflowne are this to him literally, since even after Van kills all of them their spirits intervene in order to protect Dilandau and allow him to escape.
Hak, the Four Dragons, and various other characters to Princess Yona in Yona of the Dawn, and vice versa.