A person (usually The Hero) has her or his motivation built around protecting another person. Most of the time, it's his or her Love Interest, but it could also be her or his little sibling, or child, or parents/parental figure.
In Anime, the sentence "Zettai Ni Mamoru!" is likely to be used - if not spoken aloud, then thought. Depending on context, this can mean anything from a literal "I'll protect you from danger" to a more comradely "I'll watch your back" to akin to a Love Confession, i.e. "I'll always be there for you." Because of this, expect the recipient to respond with a Luminescent Blush regardless of what she says.
Almost every time, some sort of complication will arise from this situation.
The protected person is capable of looking after her or himself. The protector has a hard enough time keeping up with them, much less being of any use, and will often chastise themself for failing to live up to his perceived duty as protector. Inevitably, however, the protected will get into trouble, at which point the hero's Heroic Resolve will manifest and s/he'll become suddenly competent as s/he leaps to save her.
The protector fails to protect the protectee, resulting in the latter's death. This will be his or her Greatest Failure. In this case, the relationship is more likely to be part of the backstory. It might have been the former protector's Start of Darkness, as s/he went over to The Dark Side while seeking revenge, or went mad while seeking to bring the protected back. Or maybe the protector just cracked and turned into a Death Seeker. If the protected person survived despite his failure, the effects will be less severe, but all of the above can still occur — there's just a better chance of redemption later on. If the protector has to protect multiple people, and fails at doing so, then they're effectively a Doom Magnet and tend to suffer Survivor Guilt.
The protected person is normal, and the protector is not... and the main reason the protected is even in danger is that someone is hunting the protector. Thus, the protector feels compelled to part with the protected, or run away from any other responsibilities to keep her safe. It almost never works. Note that most of the examples in It's Not You, It's My Enemies do not apply here — for it to qualify, protecting that person has to be the character's main driving motivation — so, for example, Spider-Man wouldn't qualify, since he knows he has a greater responsibility to protect the entire city, and the world. (And let's not even mentionOne More Day...)
The protector protects their person, but at the cost of his/her life, or nearly so. If the protector dies, it could be a Start of Darkness for the protected, particularly if the protected person has any kind of power. If the protector survives, it's more likely to work as a Berserk Button, but it might also compel the protected to try and push him/her away so s/he won't get hurt again.
The protected person turns out to be The Mole, or just plain bad news. The protector likely won't see this, even if other try to tell her/him - and it's fairly likely that s/he won't realize it 'till the protected person finally stabs him in the back. It can turn out in a lot of different ways, but it's usually quite tragic. If the protector kills their (former) protected in response, it'll probably turn out that the traitor was acting under duress, and the protector will be swallowed by grief and regret. If the protector dies by the traitor's hand, s/he'll probably realize that s/he really did love her/him, and be swallowed by grief and regret. But if the protector continued to insist on protecting the protected, even after s/he's shown her True Colors, s/he might realize that s/he really does love him before s/he kills her/him, and be redeemed by love...
'Protecting' turns out to be more complicated and morally ambiguous than it sounds. Maybe the protected person has some sort of weird power that makes a lot of people fear them, or maybe the very layout of the world makes it impossible for them to live in peace. The hero may be forced to fight and kill more-or-less innocent people who attack the protected person out of fear, or orchestrate rebellions and war in order to change the world for their love's benefit — either way, there's bound to be a lot of victims, and the 'hero' will start to look less and less heroic...
In the past, the protected person did something for the protector. Either saved her/his life, pulled her/him out of trouble, gave her/him something s/he needed, befriended him/her when nobody else was willing to, etc. So the protector cherishes said moment in which the protected was there for her/him, feels indebted to them, and swears to protect them as a way to pay them back for their help. Debts must be paid, after all... even if it involves pretty much going to Hell and back to not owe that person anything.
Naturally, not all cases will fit neatly into one of these categories, and of course, there are cases where there isn't really any major complications - just someone protecting another person from the Monster of the Week, for as long as it takes. And sometimes the declaration is mutual, and you get a Battle Couple.
Related tropes are numerous. Effectively, it is a subtrope of The Protectorate and The Champion. Bodyguard Crush is when the protection comes before the emotions. The Dulcinea Effect is when this absolute dedication pops up all of 10 seconds after he first sees her. Mama Bear and Papa Wolf are when it's a parent protecting their child. This trope is part of the job description for Lady and Knight.
Warning! Some of these examples will be spoilers solely by virtue of which 'type' they get placed under!
Declaration of Protection
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The protected person is a Badass themselves.
Anime and Manga
Kekkaishi: Yoshimuri has sworn to protect Tokine, but while he has a great deal of raw power in the art of Kekkaishi, Tokine is vastly superior in terms of skill and refinement - so he can barely keep up with her, and usually ends up needing her help in a fight. (In truth, they're pretty evenly matched, but she's too proud to admit it and constantly belittles his lack of skill - which he takes to heart, causing him to often convince himself that he was no help at all, even though his raw power was as important in defeating the Monster of the Week as her technique.)
Claymore could be interpreted as having at least two of these.
In Dragon Crisis!, Ryuuji does this for Ai. She is able to read his thoughts due to magical earrings she was wearing at the time. Although she learns that he's scared, he still tries to protect her, long enough for the Big Damn Heroes to show up to help him.
Gin's resolve was strengthened by this trope. Gin and Rangiku met as street urchins and when Rangiku was attacked by Aizen's men, he vowed to become a shinigami so she'd never cry again. He failed to achieve his revenge, his last sight before he died being Rangiku crying over his broken body.
Katagiri vowed to protect the more powerful Ryuuken no matter what the cost, and that no matter what happened she would never, ever leave his side. A promise Katagiri could not keep as she was eventually murdered by The Emperor. Souken once strongly implied that Ryuuken's behaviour is based on a similar declaration he's made to ensure his and Katagiri's badass son, Uryuu, does not share his mother's fate.
Tia Harribel towards her Fraccion. In the wild, she lived as close as she could to a life of non-violence and starvation to defy the Social Darwinist laws of Hueco Mundo. Since then, she has been motivated to fight by a desire to keep her few friends from dying... and these friends are her Amazon Brigade.
Shuuyu has this problem in Ikki Tousen. He's a skilled fighter, even amongst his fellow 'reincarnated Romance of the Three Kingdoms warriors', but he doesn't have quite the same level of world-shaking power as his Bodyguard Crush, Sonsaku. The fact that Sonsaku is Oblivious to Love and a fight-loving Blood Knight does little to improve his situation - her first reaction to him declaring his intent to protect her even at the cost of his own life, is to ask if he's eaten something strange...
The eponymous character of Kenichi: The Mightiest Disciple has sworn to just about everyone in earshot that he'll become strong enough to protect the Love Interest, Miu. Naturally, since she's the granddaughter of the man who owns the dojo that Kenichi attends, she's a far better fighter than him.
Guts from Berserk falls into both the first and second categories listed above. He did everything he could to save Casca during the Eclipse, up to and including chiseling off his own left wrist, and she was still raped in front of him by Femto. She survived, but her mind was completely gone. Her fate is most of what fuels his post-Eclipse Unstoppable Rage during battle and continuous Roaring Rampage of Revenge. Later, when Guts puts said roaring rampage on hold, his top priority is to protect Casca above all else - even from himself.
While rather understated (as in it's clear to the reader, but he hardly shouts about it) it's plainly one of Soul's motivations for protecting Maka. He insists its because it's his job as a Weapon. Fits whether or not you view them as a canon couple. Jackie shows a similar angle towards Kim with aforementioned connotations. Though the basic sentiment extends to pretty much every Weapon/meister team and works both ways.
While Akira and Mina generally fit in this category, it's complicated by the fact that they're a match for each other. Though to be frank if Mina had to watch her own back all the time she'd have absolutely no time left to rule, and Akira has "protecting her" in his job description.
Another interesting point is that if we reverse the roles, then Mina herself clearly fits the last category. Akira is her anchor in the merciless world around her, and their childhood promise is the main thing that holds her still on the Slippery Slope.
In Pokémon Special, admittedly, it takes only a little while for Lady Berlitz to become an Action Girl, but nevertheless, Dia and Pearl swear to protect her with their lives after they learn the truth about the whole situation surrounding her though Dia did have a good idea of what was really going on the whole time and when her real bodyguards get banished into the Distortion World.
In Code Geass, Lelouch does this for Kallen first by lying to her about using her in order to have her spared from going down with him when the Black Knights turn on him, and later by keeping her locked out of the Zero Requiem.
Deconstructed in Ayashi no Ceres. Shiso and Ceres have an Inter Species Romance-type marriage, with him as a human and her as a Magical Girl of sorts. After they're attacked and he cannot help her fight off their attackers, Shiso decides that he doesn't want to be The Load and asks Ceres to give him a part of her powers, so he can fulfill this trope. But said powers drive him literally crazy for Ceres, with dire consequences.
Angel Beats! subverts this with Hinata and Yuri/Yurippe here seeing as they're both immortal.
In Sekirei, Minato often ends up doing this for the Sekirei he eventually becomes their Ashikbabi for. This helps win over many of them, such as Musubi, his first Sekirei, Kusano, and later Tsukiumi.
Kurando's primary motivation in Popcorn Avatar is to protect his twin sister from the Asura and maintain a world where she can safely live in.
In X1999, YuzurihaNekoi verbally invokes the trope in the manga as she and ArashiKishuu speaks about the guys they like. The dude Yuzu wants to protect? Shiryuu Kusanagi, a man who not only is more than twice her age, but is a member of the Japanese Self-Defense Forces and quite the badass among the Dragons of the Earth.
Similarly, Lady of War Arashi's suitor is Sorata Arisugawa, a Religious Bruiser and fellow Dragon of the Heaven. Upon seeing her up close for the first time, Sorata decided to protect her. This is because he's prophesied to die to protect a woman, he knows it clearly, and he thought he'd at least die for a pretty girl and chooses Arashi at first due to her Aloof Dark-Haired Girl traits. And from then on Sorata gets Arashi to gradually defrost, and they fight together... and get together. But things go From Bad to Worse.
In Girls und Panzer, Maho's ultimate goal in living up to the standards for the Nishizumi heiress is so that her younger sister Miho can live and do tankery as she wishes, without having to take up the responsibility in her stead. It takes six years, from near the end of the prequel manga to the last episode of the first series, but Miho ultimately finds a way of Tankery that she can do, and Maho is very pleased with this.
Maho: As long as I'm the heir to the Nishizumi school... Miho will be free.
Mahou Sensei Negima!: Asuna is Negi's ministra magi and thus responsible for protecting the Squishy Wizard while he casts his spells. In response, Negi pushes himself to become stronger so she doesn't have to protect him and put herself in danger. While competing in Mahora Fest's Tournament she loudly declared to Negi that she was plenty powerful enough to protect him. Then she became profoundly embarrassed because she was fighting at that moment and the entire crowd interpreted her declaration as a love confession.
Might also count as an inverted case of 'it got complicated'... Saitohimea has been on the edge of a Suicidal Cosmic Temper Tantrum due to immortal loneliness before, and her relationship with Taito is all that's keeping her (relatively) sane at the moment. Hence, MAINTAINING their relationship is of no small interest to a number of organizations with a vested interest in the continued existence of the universe.
In Saki Achiga-Hen, Yuu's younger sister Kuro often protected her from bullies while the two were young, who picked on her for wearing warm clothing in summer due to being chilled easily. During the quarter-finals, however, when Kuro comes up to Yuu in tears over losing so many points, Yuu comforts her and declares her intention to win the points back, mentally expressing her determination to stand up for her sister.
Yuu: "Kuro-chan has always been the one to save me... But today, it's Onee-chan's turn to fight!"
In Naruto Rock Lee promises to protect Sakura during the second exam of the Chuunin Exams, risking his life to protect her even though they aren't even teammates and nearly dies protecting her, even pushing his body to attack when it was deemed impossible for him to move.
One Piece: Ace swore to protect his younger brother Luffy, both out of love for him and as a promise to their other "brother" Sabo, who he thought was dead. Indeed, Ace spent most of his childhood as Luffy's protector, and ultimately gave his life to protect him during the Paramount War (which in itself was a tragedy, as Luffy spent most of the war trying to save Ace from execution). Years later, Sabo, the third brother, reveals that he is alive to Luffy. Later on, as he battles Admiral Fujitora, he told his opponent that if Luffy were to ask for his aid, no matter where he was in the world, he would drop everything and be at his side in an instant. There is a quick flash to his reaction to Ace's death, and the look of pain on his face makes his reasons clear: he lost Ace, and he'll never know whether or not his brother could have been saved if he had acted. He won't take that risk and lose Luffy as well.
In Suzanne Collins' The Hunger Games, Peeta devotes himself to this cause for Katniss, who spends most of her time in the series kicking asses and taking names. He generally devolves into The Load because he saves Katniss' life by fighting off Cato and gets a sword to the leg for his troubles. In the second book it's less heroic, but getting electrocuted would slow most people down. In the third book, the Capital, aware this trope is in effect, turns it against them both in a sadistic way: brainwashing Peeta to assassinate her on sight
River Tam in Firefly is a subtle distinction; She really did need and want her brother Simon's protection, but that was because she was so erratic that she needed a stabilizing influence. She needed his love more then his protection. Despite her abilities and combat training she's still scared and confused about herself and the world around her. And Simon, despite possessing a Papa Wolf instinct to rival the Captain's own, had an unimposing appearance and soft-spoken nature unless he needed to protect someone (like a patient endangered by an incompetent surgeon).
River: You take care of me. You've always taken care of me! *Beat* My turn.
Another Joss Whedon show: nearly everything Paul Ballard does on Dollhouse is to protect Echo...even before he actually meets her.
A third: Buffy the Vampire Slayer runs on this trope. Most if not all the main characters are committed to protecting each other, as well as a good number of guest stars as well.
In Chinese Paladin, Xiaoyao marries and swears to protect Ling'er. He takes this vow very seriously although initially she is far a more powerful combatant than him and all she needs is his emotional support. However, as the series progresses he Takes A Level In Badass and it becomes Just That Simple between them.
On The Vampire Diaries, Stefan, who is the hero, devotes his immortal, undead life and makes constant sacrifices (including sacrificing himself or risking his life) in order to protect and save Elena, the heroine (or more like damsel in distress) from possible harm and dangers of the supernatural in the town of Mystic Falls.
In "The Power of Three" he tells a levitating alien cube that earth and its people are precious to him, and that he will defend them to the death, if need be.
In The Bells of Saint John, after he recovers Clara Oswald's consciousness from a Spoonhead android, the Doctor sends a message over the Wi Fi network, "Under My Protection".
He declares this again in "Nightmare in Silver" about the young children that the Cybermen have begun to convert.
The Tenth Doctor pulls this off in his first adventure, "The Christmas Invasion":
The Doctor: By the ancient rights of combat, I forbid you to scavenge here for the rest of time. And when you go back to the stars and tell others of this planet, when you tell them of its riches, its people, its potential, when you talk of the Earth, then make sure that you tell them this... It. Is. DEFENDED!
Arado Balanga from Super Robot Wars Original Generation 2 is a pretty good Mecha-Pilot, objectively speaking - but as a graduate of 'The School', he's at the bottom of the heap - especially compared to his partner, Seolla, who's one of the best, period. But he's sworn to protect her, and protect her he does - whenever she ends up in a pinch, he goes Crouching Moron, Hidden Badass, whether that means fighting off an otherwise-supreme opponent, or taking a bullet for her. Sometimes both.
An odd twist on this for both Knights of the Old Republic if you're playing female. In the first, Carth knows damn well you could skewer him on a lightsaber and not muss your hair. Still does not stop him from admitting that trying to protect you is his reason to live after Saul Karath dies painfully. In the second game, Atton's overt about it. Bao-Dur is more subtle, but still frighteningly loyal to his "general." In both Carth's case and Bao-Dur's, the "protection" they offer is more or less trying to act as the Morality Pet for a female Player Character in order to help prevent her from falling to the Dark Side.
Persona 3: It's a gender reversal, and it's after everything is over (until FES), but Aigis sort of qualifies with her wanting to protect the main character...who can certainly fend for himself.
AkihikoSanada is a straight example across all of P3's releases, with his resolution (which brings about his Persona Polydeuces' transformation into Caesar) being his resolve to protect all of SEES, but he plays it straightest if he becomes a female protagonist's Love Interest in the Playstation Portable version of the game. He also has ties to the "failed to protect" category, as his younger sister Miki died in a house fire when he was a child, and later on, his childhood friend and Guest Star Party Member Shinjiro is murdered.
Tales of Graces: Main characters Asbel and Sophie argue over who gets to protect whom, until Asbel challenges Sophie to a footrace to settle the matter...and loses. Sophie saves the group from certain death more than once throughout the game, and is canonically one of the most powerful fighters in the party. ( Halfway through the main arc, when Asbel is given an opportunity to defeat her in single combat).
After Bartlett is shot down in Ace Combat 5: The Unsung War protecting Edge she becomes very determined to protect you.Bad Ass? There's a myth about the Demons of Razgriz that applied to the Wardog squadron, and they may indeed be literal demons if one mid game mission is anything to go by.
Fate/stay night.Emiya Shirou. He does this twice. In Fate, he does this to Saber, despite Saber being motherfucking King Arthur. (To be fair, he HAD no idea who she really was AND had watched her get totally curbstomped by Hercules.) Fortunately, he's called out on this and eventually it gets deconstructed in the Unlimited Blade Works route (the second example), where it turns out that Shirou has a massive case of Survivor Guilt that causes him to totally disregard his own safety. Heaven's Feel also plays with this trope by having Sakura be, from Shirou's point of view, totally helpless... then you get closer to the end and she turns out to have an absurdly powerful Superpowered Evil Side. Shirou promptly modifies his definition of "protect Sakura" to mean "get rid of Sakura's Superpowered Evil Side."
Kerry from DSBT InsaniT makes it a point that no one shall harm Shawn, although the latter is certainly capable of protecting himself.
Kerry: Nobody harms Leader Shawn!
Avatar: The Last Airbender: When Sokka was still feeling guilty after losing his last girlfriend, he decided to make sure nothing happened to his other love-interest Suki. Of course, being an Action Girl, Suki is mostly bemused and slightly insulted by this- and eventually reveals the only reason she went with them was to protect Sokka.
Failed To Protect Them
Anime and Manga
Code Geass gives us Suzaku and Euphemia. Initially a straight and uncomplicated 'knight and princess' relationship, Euphemia's death at the hands of Zero pushes Suzaku off of his white horse, driving him into much more questionable actions, such as selling out his oldest friend to his worst enemy or releasing a powerful nuke in battle... and, in the end, he winds up supporting one of the bloodiest tyranies in history. Led by, you know, his oldest friend.
Mahou Sensei Negima!: Konoka and Setsuna's dynamic falls most closely under this but covers several variants. While Setsuna is heavily motivated by a backstory incident where she failed to protect Konoka from drowning, she also spends a lot of time trying to protect her by hiding the truth about magic from her, thus making it a case of It's Not You, It's My Enemies. Later on, she nearly dies while trying to rescue Konoka. Ultimately her absolute dedication to protecting Konoka comes back to the old childhood 'close shave' where they were both lucky to survive. (Whether Setsuna's dedication edges into the territory of Schoolgirl Lesbians is left as an exercise for the class.)
Karasu of Noein is basically built around protecting Haruka, typically telling his opponent something along the lines of, "I will protect her with my life." This is because, in his opinion, he failed to protect her future self, though it is implied she willingly did a Heroic Sacrifice.
Ikumi from Infinite Ryvius lost his sister years ago, and was severely traumatized by it... which makes sense, seeing as she committed suicide because of their illicit Brother-Sister Incest relationship. So he's sworn never to fail to protect a woman, particularly one he cares about. When his new girlfriend, Kozue, is attacked and scarred while he was out fighting, it becomes his Start of Darkness as he decided that Utopia Justifies the Means - he'll do whatever it takes to protect her from now on. It... doesn't end well. Either way, that means he fits this subtrope TWICE OVER. Poor guy.
Game of Thrones: Faithfully serving Renly Baratheon as a member of his Kingsguard is Brienne of Tarth's quiet way of expressing her (unrequited) love for him.
The X-Files. Mulder and Scully could fall under almost all of these, but this one is closest. While Mulder was always protective of Scully her abduction in season 2 sent him into overdrive. After her return becomes obsessed with keeping her safe. He ditches her several times, much to her annoyance, and asks her to sit out on cases for fear of her life. This makes it obvious that the one way to hurt him is to hurt her, and so this protective streak becomes even more pronounced after she is kidnapped many times more. Eventually, this turns into "I Owe Her My Life", since Scully is no slouch in the protecting and rescuing department.
Spike showed hints of this after Buffy died in the episode "The Gift".
Zig-zagged in Dissidia: Final Fantasy at first Cosmos's warriors (especially the Warrior of Light) make it their mission to protect her from Chaos and his henchmen. In the end she willingly let's herself die in an effort to save them from the cycle of death and rebirth they've been caught in.
Prince of Persia (2008) has the prince play out this trope. He doesn't really care about the lost kigndom, or the Big Bad, or any of that - he's only fighting because Elika DOES care, and she's gonna put herself in harms way with or without him. In the end, she gives up her life to stick the Seal back on the Can of Evil, and he immediately snaps and tears the lid back off to ressurect her again. She is not happy about that.
Sorta invoked in The King of Fighters. One of Kyo Kusanagi's main motivations in the Tales of Ash saga is to make Ash Crimson pay for attacking Chizuru Kagura, his friend and fellow member of the Sacred Treasures Team, and stealing her Magic Mirror. It's really not helped by how Kyo couldn't do anything to stop him.
In the Baldur's Gate series Minsk has a "Witch" called Dynaheir. It's his duty to protect her, and if you do not resolve his Side Quest to save her, he'll leave your party. The sequel kicks off with you (And Minsc) in captivity and him lamenting how he failed to save Dynaheir, her having died off screen. Somewhere in the game Minsc can choose a new "Witch" to protect, out of two other potential party members.
In Death Note after Sayu gets kidnapped by the mafiaLight deliberately sets things up so that he can't kill her without incriminating himself.
Film - Animated
In Frozen, this is why Elsa isolated herself from Anna and remained cold and aloof from her as an adult. When she unleashed her ice powers at the coronation, she attempted to flee the kingdom to protect her subjects in general and Anna in particular, not realizing that she locked her kingdom in Endless Winter.
Film - Live Action
Iron Man 3 initially seems like a simple case of 'beat the bad guys to protect her, with Tony Stark is building new armors in order to protect Pepper Potts. (As well as dealing with his PTSD and insomnia after the events of The Avengers.)
"I have to protect the one thing that I can't live without. That's you."
A twist on this can occur in Suikoden II. The Hero has somehow wound up leading La Résistance against The Empire, and his adopted sister has, naturally, wound up getting dragged along for the ride, despite being just about the only playable character NOT listed in the stars as one of his 108 destined companions. When she finally admits that she'd prefer to live quietly together, you have the option of agreeing with her, running away from your army and responsibilities along with her, thus demonstrating an alternate solution to the problem. The result is somewhere between a Non-Standard Game Over and a... well, Not All That Bad Ending. You actually DO get away, and live quietly together in a cabin in the woods somewhere far from all the war and strife... but of course, La Résistance fell apart, The Empire triumphed, and all your other friends probably died.
Sluggy Freelance, of all places, has an example of this. Torg may have a lot of diversions, but there's only ONE thing he's actually SERIOUS about - and that's protecting Zoe. In an alternate universe overrun by demons, he soon gave up on trying to run La Résistance (since everyone there are Perfect Pacifist People), and instead just concentrates on keeping Alt-Zoe fed and away from trouble. Back in the real world, he forces her away so she won't become a target for Stalker with a Crush Oasis. With... mixed success.
Taking a Bullet for Them
Katsa in Graceling has a more mild case as she tries to protect Princess Bitterblue from the child's ownfather. To do so she has to take the young child across mountains nobody ever survived before, which is easier for her than most people as she's graced with survival. In the end of the journey, while the princess manages to be okay, it's Katsa who ends up getting sick, for the first time in her life. She does get better, though. Prince Po, who was also trying to protect her, went to go challenge her father head on and was nearly killed. He survived, but was struck blind and had to stay behind while Katsa and Bitterblue left to escape together.
Steve Jinks, Warehouse 13 has an interesting combo of three, He has a dead big sister that he sees in Action Girl Claudia, his partner, after she saves his life in the most hilarious way possible. This lead him to decide to go undercover to protect her because he couldn't save his own sister, which in turn, gets him killed. He gets better.
This is one possible outcome at the end of Dragon Age: Origins: if a female Grey Warden has a romance with Alistair, he will refuse to allow her to sacrifice herself to kill the Archdemon, taking the choice out of her hands and doing it himself unless she takes the precaution of leaving him behind at the gates before heading to the last battle.
Resident Evil 2, and quite literally, in the case of Leon and Ada. Crossed with The Mole scenario.
The Big Daddies of Bioshock are willing to die to protect the little sisters.
Happened in Girl Genius when Lars — a traveling performer previously renowned for his cowardice — threw himself in front of a sword to protect the eponymous protagonist, Agatha. He died, and she went nuts, temporarily going into full Blood Knight mode, and attempting to basically take on the entire army of Wulfenbach singlehandedly. (Well, with a little help from her Mecha-Mooks...) She got over it, with a bit of chemical assistance, but now seem hell bent on making sure nobody else dies for her. (Cue Gil and Tarvek winding up in one of the above-mentioned 'She's an Action Girl' situations, as she works harder to protect them than the other way around.)
The Protected Was The Mole
Anime and Manga
Rurouni Kenshin. Specifically, the 'backstory'. Back when he was still an assassin, Kenshin fell in love with a woman named Tomoe - who was actually a spy planted by the Shogunate to find his weakness and eliminate him. The details are awfully complicated, but after The Reveal, she ends up dead along with her employers, and Kenshin swears never to kill again...
Revolutionary Girl Utena probably fits best under this, and is a very genderqueer example, what with Utena Tenjou deliberately taking on the persona of a Fairytale Prince. She fights solely to protect the Rose Bride, Anthy Himemiya... but at the end, it turns out Anthy was in on the whole thing, and was in fact following Akio's will, from being involved in his manipulations behind the scenes to doing everything he told her to . . . EVERYTHING. It's really, REALLY not helped by Utena realising that she had been taking the persona of a prince less to help Anthy and much more to feed her own ego, not taking Anthy's thoughts and beliefs in consideration. After this, Utena is severely disillusioned, becoming a Knight in Sour Armor, but comes to forgive Anthy after realizing just how much torment she's been in for hundreds and thousands of years, and resolves to truly save her from more suffering.
In Kieron Guillen's run on Uncanny X-Men, Colossus became obsessive about protecting his sister Illyana, to the point of losing his humanity(not to mention the love of Kitty Pryde) by taking on the Juggernaut's curse. Toward the end of Avengers vs. X-Men, Illyana revealed that she was Evil All Along, and had manipulated her big brother into becoming the new Juggernaut in order to make him as much of a monster as she had become. Let's just say Colossus no longer feels any need to protect his "little snowflake"; he has in fact vowed to kill her if their paths cross again.
Resident Evil 2, and quite literally, in the case of Leon and Ada. Crossed with Take a Bullet scenario.
Protecting Gets Complicated
Anime and Manga
Obviously, Code Geass, where Lelouch's desire to give his younger sister, Nunnally, a 'world of peace' leads him further and further down the path of darkness. When she finally finds out what he's been doing in the name of helping her, she's horrified and becomes an easy prey for Schneizel's manipulations.
Toshiro Hitsugaya and Momo Hinamori in Bleach. As a kid, Momo was the only child in the neighborhood who wasn't afraid of Hitsugaya due to his burgeoning ice powers. As they became Shinigami, he swore that he'd protect her and pretty much kill anyone who ever made her bleed a little... But when Momo was horribly broken and then almost killed by the man she loved, Sousuke Aizen, Hitsugaya tried to avenge her, and was almost killed himself. When Momo got better and reclaimed her Action Girl credentials, she and others were horribly defeated by Allon. And then... Aizen used his powers to trick Hitsugaya and other captains into stabbig Momo to almost death, thinking they were stabbing him.OUCH, it definitely got complicated.
Also, after Karakura is attacked and Orihime is seriously injured, Rukia brings Ichigo to her to apologize for not having been able to shield her, and then Ichigo makes the promise that he will protect her from then on. Considering that later she was forced into a cruel Sadistic Choice that involved her going to the villains's HQ and Ichigo then goes to rescue her, it got complicated and then some. This trope was hammered into the ground when Ichigo - who'd just gotten a hole blown through him by Ulquiorra's Cero Oscuros - Hollowfied in response to Orihime screaming his name, mortally wounded Ulquiorra, and impaled Ishida before turning to her and saying "I'll protect you!"
When Homura Akemi in Puella Magi Madoka Magica was caught in a witch barrier, Magical Girl Warrior Madoka Kaname saved her. She then vowed to help her and protect her in return. She failed, however, and Madoka died. Then Homura made her contract with Kyubey, and specifically hoped to protect Madoka and avert her death. She failed each time, but kept on trying...
Things start off simple for Melan in Brigadoon: Marin and Melan. His job is to protect the Creis, who happens to become his friend and eventually his Love Interest. Then it turns out that she's not the Creis after all, and protecting her means taking time out from his REAL mission.
The Sword and Sorcerer team Shannon and Raquel from Scrapped Princess are solely motivated by their oath to protect the eponymous princess, Pacifica. Shannon is the one that most closely match the trope, being a guy and all - since they were raised as siblings, his affection for her is more big-brotherly than romantic, but they are Not Blood Siblings, so it gets... ambiguous at times. As for complications, well, according to a supposedly-infallible Divine Prophecy, Pacifica is 'the poison that will kill the world' - specifically, she will destroy the world when she turns 16. As a result, virtually everyone wants to kill her, either out of fear, religious fervor, or greed for the huge bounty the kingdom has placed on her head. And Shannon is stuck trying to protect her from all that, preferably without killing anyone because he knows that Pacifica would blame herself for any lives lost...
In Death Note Rem forces Light, under direct threat of death, to adopt this mindset towards Misa in regards to his plans during the events that lead up to the Yotsuba arc.
The Wonder Woman graphic novel The Hiketeia revolves around this. Wonder Woman gets bound by an ancient Greek rite (the Hiketeia) to protect a young girl, on punishment of painful death by the Furies if she breaks the oath. Unfortunately, Wonder Woman hadn't realized the girl is a murderer (with a pretty damn solid justification) on the run from Batman who's determined to bring her to justice. Batman won't back down and Wondy can't either, so they end up duking it out until the girl commits suicide.
In Tanz der Vampire, Alfred is searching for Sarah and vows to protect her, at any cost.
I'll be tough, I'll be sly, There is nothing I would not do or be For Sarah I will steal, I will lie There is nothing I won't try To find and save her.
In NieR, the eponymous (assuming you use the default name) character's overriding desire to protect his daughter, Yonah, initially seems perfectly simple... and HE probably never realizes it was anything but. The player, on the other hand, will soon realize - by the second playthrough if not sooner - that he's been doing a LOT of really questionable stuff in order to protect and save her.
In Sands of Destruction has a gender flipped version of who to protect. Toppi grows an attachment to Kyrie because of his positive ideals and swears to protect him. It's complicated in the since that Kyrie is a Person of Mass Destruction and has the power to destroy the world and Toppi really doesn't want that.
All main routes of Hakuōki lead inevitably to this trope thanks to Chizuru spending a good four years in the constant company of the captains of the Shinsengumi, all of whom, aside from Sanan, develop protective feelings toward her and go out of their way to keep her with them and protect her, even though she's The Load and the Shinsengumi's situation becomes increasingly dire as events unfold. In Okita Souji's route, her Evil Twin Kaoru purposefully exploits the trope with malice aforethought to make poor Chizuru suffer by repeatedly creating situations that inspire Souji to protect her at his own expense.
It's kind of a plot point in the KOF: KYO continuity. Ever since he was a kid, Kyo deeply wanted to protect his loved ones (in fact, he promises his mother Shizuka to do so when he's as much 8 years old), and that turned out to be one of the reasons why he was so powerful... but after gradually forgetting it in the middle of his turbulent life, he got Brought Down to Normal and then susceptible to being Brainwashed and Crazy by Orochi. Remembering how he began to fight as well as this purpose is what lets Kyo recover his lost powers and shake off Orochi's influence: this is sealed after his Warrior Therapist dad Saisyu undoes a staged Hostage Situation that he built to help Kyo and releases Yuki from an Unwilling Suspension. As Kyo rushes to catchher, his inner thoughts pretty much spell this trope.
Also invoked by Adelheid "Adel" Berstein. He has sworn to himself to be the protector of his younger sister Rose, who also is his only living relative after their dad Rugal's death, and even when he seems to be an Aloof Older Brother he will NOT stop when it comes to shield her from any harm. The main reason why he became Heidern's source of information in XIII was his worry about how oddly Rose was acting around him — and with good reason, since she was Brain Washed by Those of the Past. When she's released from their mind control and collapses, Adel immediately rushes towards her to catch her in his arms.
Other routes also touch the trope, either criticizing it or reaching a compromise between protecting/helping a girl and letting her be when needed. In example, Emi's path shows that Emi does need emotional protection due to refusing to make any deep relationships after her father's death in the accident that claimed her legs, and as long asa Hisao doesn't overdo it, he can protect her broken emotions; and in Rin's it turns out that Rin needs both protection (since she's under very heavy pressure from her well-intentioned but VERY pushy art teacher) and to be understood (because she has big problems on expressing what she feels if it's not through art)
In Little Busters!, Kyousuke's reason for most everything he does in the game is to protect Riki and Rin. Specifically, to make them stronger so they can survive without him after a bus crash claims everyone else's lives. Since he won't be around much longer, protecting them also means making sure Riki is able to protect Rin and doing everything he can to encourage him to give himself over to this view completely. In the end, Riki also vows to always protect Rin.
In The Sims 3, if you have supernatural installed, your Sim can make a promise to protect another, especially if said other is a child or romantic interest.
In Girl Genius, Agatha Heterodyne, Gilgamesh Wulfenbach, and Tarvek Sturmvoraus trade off on this relationship: they're all more or less madly in love (emphasis madly, also "mad"ly), and as heirs to the three most important dominions in Europa and strong Sparks they're generally quite badass. None of them particularly like the others' protective behavior, but it's hard to blame them given the stakes. Of course, just to twist the knife, there are slaver-wasp-related reasons why they can't be together...
Silver in Above Ground wants to protect Lilith and help her return home safely. But he's in complete denial about it all because he knows the desire to protect is an unnatural bewitchment rather than a real feeling. It doesn't help that his pack is in a dire situation and they need money that Lilith could provide if they ransom her off to the Guild.
I Owe Them My Life/Reputation/Health/etc.
Anime and Manga
Hayate the Combat Butler: Hayate (the butler) to his mistress, Nagi. She saves him from his miserable life by paying off his debt to the Yakuza and giving him a home in her mansion and a job as her butler, with a promise to pay her back over years of work. But it never was about the money - the amount means nothing to her absurd wealth. She simply likes him from the start, while his gratitude quickly compounds with a deep affection and interest in her well-being.
There's also the residual guilt from initially wanting to kidnap and ransom her. He was in a desperate enough situation — thanks entirely to his Abusive Parents — that this isn't a Moral Event Horizon, but he still felt terrible about it after he came to his senses.
While Kenshin's devotion to Tomoe in Rurouni Kenshin fits in She Was the Mole, his relationship with Kaoru Kamiya fits here. She gives him a place to stay, emotional support, and the chance to stop Wandering the Earth; as such, he decides to protect her and their friends.
Also by Watsuki, Kazuki from Busou Renkin gave this speech to an opponent (who also gave the same speech), when asked why he was protecting Tokiko. He said that it was because of Tokiko that he was still alive, so he would protect her and fight in her place. Kazuki's situation is actually a combination of type 1 and type 7, since Tokiko is a very capable Action Girl.
In magico Shion is decided to save Emma from the curse of Echidna and faces really dangerous tasks with his extremely powerful magic that he obtain just to do so because... she gave him some bread
In The Matrix slashfic Bringing Me To Life is a combined example of 1, 3 and 7. # 1 is - Smith, Love Interest, says he's going to protect Max from his abusive father because in a past life Max saved his life. #'s 3 and 7 - Max is also recovering his Past Life Memories and, slowly, regaining his badass status.
Rafael toward Skylar in Gives Light, but in this case, it's "I will protect him." Rafael's father, a serial killer, killed Skylar's mother many years ago, and Rafael feels responsible by proxy.
Tin Man A cross of "I owe her," "It gets complicated," and Living Macguffin get involved with the gentlemen who decide to travel with DG. Raw is pretty straightforward. She's a good person, and she was responsible for rescuing him from a nasty death. Glitch comes with because it's preferable to be confused and with someone than confused and alone. But it turns out that he was her mother's trusted and devoted adviser. Remembering who he was and learning who she is just makes him more determined to help. Cain's a mess. He agrees to escort DG and Glitch to Central (Emerald) City because he does owe them that much for freeing him, but the Mystic Man, his former boss, orders Cain to protect DG at any cost - and Cain's too damn honorable to disobey a direct order, especially as it turns out to also be the Mystic Man's Last Request. By the time it's discovered that DG is inadvertently responsible for the Witch's reign of terror, they've become too much True Companions to care.
It's Just That Simple
Anime and Manga
Murder Princess is a straightforward example. It takes Fallis a while to realize that she wants to protect the sweet, gentle (and very, very helpless) Princess Alita, but actually doing so never gets any more complicated than cutting down anything that threatens her or her kingdom.
Blue Exorcist has the relationship between twins Rin and Yukio when they were younger. Yukio was bullied and Rin went to beat the bullies up just because they were picking on Yukio.
In Field Of Innocence, Lu Ten promises his younger cousin Azula that as long as he's there, he'll always protect her. As he is Doomed by Canon, you can guess how well that works out.
Memory, Sorrow, and Thorn: Simon has a very difficult time spitting out his affection for Rebellious Princess Miriamele, despite the fact that he has a crush on her. Part of it is that he feels that she's so far above him socially that his desires will remain unrequited, although he fantasizes about rising to her station or her falling to his. Of course, they keep getting thrown together in perilous circumstances, and after he's knighted, he resolves to become her champion and protector as a way of dealing with the situation. In the end he discovers that it wasn't their stations keeping them apart at all, but Miriamele's self-doubts and fears of being forced to assume the throne and make a political marriage. And of course he turns out to be the rightful king anyway, so he solves the problem of her entering into a loveless marriage by marrying her.
Live Action TV
Rory Williams from Doctor Who. He loves his girlfriend, later fiancée, later wife, and will do anything to protect her. A lot of the time they protect each other- for example, Rory fails at sword-fighting a fish vampire, so Amy destroys it with a mirror. However he then goes to guard her comatose body for 2,000 years, becoming the legendary Last Centurion. He seems to develop something of a complex.
Rory: And she knows that where-ever she is, I am coming for her. Always.
Castle does this for Beckett in the episode "...Boom" when a serial killer blows up Beckett's apartment:
Montgomery: You need to go home and get some sleep. Beckett: Sir, I don't have a home. Castle: Yes, you do. It's a secure building with an extra bedroom, with people who care about you. With the Federal detail at the door, it's the safest place in the city. Beckett: Thank you, Castle, but I couldn't. Castle: You can and you will.
Booth is very much this to Brennan in Bones, even before they became an Official Couple. On the one hand, his "alpha male" need to protect her tends to rub Brennan the wrong way. On the other hand, as she once observed, "I find it interesting that I'm only afraid of snakes when Booth is around to be jumped upon."
Dean Winchester (from Supernatural) has dedicated his life to protecting his brother Sam (despite the fact that Sam can protect himself, and often doesn't want Dean to risk everything to protect him).
Despite providing the quote at the top, which suggests a 'It Gets Complicated' situation, Squall of Final Fantasy VIII really DOES get it easy. In order to protect Rinoa, the girl who finally brought him out of his sarcastic, just-dont-care shell, and taught him what love was - all he has to do is beat up all the villains. Then everything is all right.
Devil May Cry 4 provides another fairly straightforward (and heartwarming) example with Nero's half of the story with his Love Interest Kyrie. Though for a while it looks like he failed.
Gungrave. The protagonist's major goals is to get revenge on the main villain, and to protect the daughter of his former love interest from the main villain. By using a lot of dakka.
In Fire Emblem for the GBA: Dorcas's wife Natalie is an Ill Girl. He wants her to get better, and so he joins a bandit group. Once Lyn knocks sense into him (and Natalie does too, if you make them talk), Dorcas decides to keep fighting for her until he gathers enough gold to buy the medicine. It's just as simple as that, for him.
In Little Busters!, Kyousuke feels this way towards Riki and Rin, and tries to encourage Riki to develop it towards Rin. In one of the final Bad Ends, when all of their other friends have died, Riki quietly promises to Rin that he will always protect her. She asks if he's proposing, and the player is given a choice whether to agree or not.
Although it has an example further up the page, Tales of Graces also qualifies here, because "protect your friends" is basically the theme of the game - it gets subverted and played straight and spun and re-used in every conceivable way. In fact the characters use the word "protect" well over a hundred times in the course of the game. This has not gone unnoticed.
Adventure Time has Simon Petrikov, AKA The Ice King, protecting a very young Marceline either during or immediately after the Mushroom War. It seems to be that he simply chose to look after her because she was scared and alone. Could arguably fall under the "complicated" category, but that's less to do with her being dangerous and more because using his powers drives him incrementally more insane. He needs to stay sane for her sake; if she weren't around, he probably would have quickly given into the madness just from using his powers to protect himself.