One very common way to establish that a male (or increasingly frequently female) character is a Nice Guy (even if the niceness is hidden under a gruff exterior) is to give them an instinctive desire to be a respectable big sibling to smaller or younger characters (in more cases female than male). These other characters may be truly younger siblings, or they may be completely unrelated, or even older, but the feeling is the same: to act as their protector.
The Big Brother Instinct serves to explain fan attraction to Moe tropes and devices as an emotional, rather than sexual, response. There seems to be something inherently comforting to an awkward male about being looked up to by a younger girl for support. Therefore, Moe can be wish fulfillment about taking a "big brother role" instead of about being a Love Interest. Rather amusingly, it's also quite often that the trope gets inverted, and the younger sibling wants to protect the older instead. Hilarity Ensues when both versions happen at the same time.
If both characters are male, the older (or in the case of twins, the physically stronger) brother will usually act as the younger/smaller brother's protector, to the point of treating him like a child, even when the younger brother is fully capable of protecting himself. This behavior can come across as loving and nurturing or as condescending and smothering. If the older brother goes to extreme, morally questionable lengths for the sake of the younger sibling, then they are a Knight Templar Big Brother.
Compare Mama Bear, Papa Wolf, Family Man, Extremely Protective Child, My Sister Is Off-Limits, Promotion to Parent. Compare/Contrast Big Brother Bully. While the two may seem mutually exclusive at first glance, they frequently overlap in both fiction and reality: the same sibling who makes their little sister's life a constant parade of pranks may have a If You Ever Do Anything to Hurt Her... conversation with her new boyfriend. Also, contrast with Disappointing Older Sibling when the character either lacks the Big Brother Instinct or is completely ineffectual at protecting the younger sibling.
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- Several from the Classical Mythology:
- Apollo, despite being the younger twin, tricks his twin sister Artemis into killing Orion because he feared this was one guy for whom she could give up her vow of chastity.
- To the great surprise of those who watched the movie Troy, Agamemnon in the original Classical Mythology and The Iliad has this in spades where Menelaus is concerned. Paris fucked with Menelaus, Agamemnon will make Troy burn.
- Speaking of Troy, Helen of Troy was once kidnapped by the hero Theseus (according to some accounts, at the age of seven) because her beauty was so great he wanted to marry her – her brothers Castor and Pollux got her back, though.
- There's also one regarding Castor and Pollux themselves. Castor was the son of Leda and her husband, the King of Sparta. Pollux was the result of Leda being seduced by Zeus, but it hardly mattered to them. The brothers were inseparable heroes until they were defeated in battle. Pollux, being a demigod, went up to Olympus, but was unhappy because his brother was in the mortal underworld. so Pollux marched down there and refused to leave unless Castor went with him, presenting the Gods with a tricky conundrum. Long story short, the brothers became the constellation Gemini.
- There's a Big Sister Instinct in the play Antigone. Oedipus' two sons, Eteocles and Polynices, were quarreling over the kingdom inheritance and eventually killed each other in a duel. Their uncle, Creon, decried that no one would bury the brother he liked less (essentially leaving him to rot shamefully in the streets) on the penalty of death, but what did the brothers' sister, Antigone, do? Defied his order and buried her brother, of course!. This ended tragically, though, with Antigone put into a tomb and hanging herself, while Creon himself had to lost his wife and son indirectly because of the grief of Antigone's death.
- Heracles had a mortal half-brother named Iphicles, the son of his mortal stepfather Amphitryon by their shared mother Alcmena. While Iphicles obviously couldn't match Heracles's divine strength or fighting skill, he was still a Badass Normal in his own right who accompanied Heracles on his military expeditions. Heracles was inconsolable when Iphicles was killed in his war against Sparta.
- More "little sister instinct". A very religious Italian woman named Angela Merici was very depressed when her older sister Gianna Maria died without rites, and prayed ceaselessly to make sure her soul would be at peace. One day, she received a vision telling her that Gianna was in Heaven with the saints. In gratitude, Angela devoted the rest of her life to teaching, religion and service to the poor, founded the Order of Ursulines, and ultimately became Saint Angela Merici.
- Happens in a couple of places in The Bible. One passage in the Song of Songs goes "We have a little sister, and her breasts are not yet grown. What shall we do for our sister on the day she is spoken for?"
- When Shechem (the man) rapes Dinah, Simon and Levi make sure Shechem (the town, including the man) pays for it dearly. Jacob isn't happy about this, but they still don't feel they did anything wrong.
- When Tamar, a daughter of David, is raped by her half-brother Amnon and thrown out, her full brother Absalom sends his servants to kill him.
- A group of GLOW villains kidnapped Little Fiji to lure her older sister Mountain Fiji out of the arena to ensure Mountain Fiji wouldn't get another victory. Little Fiji's opening rap even refers to giving her sister a call if she's in trouble.
- The Hardy brothers, Matt and Jeff. Matt as the older brother, both in story lines and Real Life, often plays this role (when he's not this, he instead is the Jerkass).
- Jay Briscoe had this reaction after Wrestling's Greatest Tag Team used an ether soaked rag to defeat Mark Briscoe in a fight without honor. ROH match maker Nigel McGuinness was able to put off Jay's vengeance but wasn't able to stop it because he could not get the obvious rule patch he wanted enforced.
- CM Punk is clearly fond of those he considers his little sisters, in Kayfabe and in Real Life. During his Chicago Street Fight with Chris Jericho, his opponent proceeded to taunt his little sister who was sitting ringside. She promptly slapped him in the face. When Chris tried to go after her, Punk pretty much lost it and beat the shit of Jericho, including putting him through the Spanish Announcers' Table with a flying elbow.
- Jericho himself qualifies for this trope in real life. At Summerslam 2016, after the controversial ending to the Orton-Lesnar match that saw Randy Orton bloodied the hard way by Brock Lesnar (and later revealed to have suffered a concussion), a furious Jericho backed Lesnar against a wall, demanding to know if that had been intentional. Repeat: Jericho was willing to get into a real fight with Lesnar if he'd hurt Orton deliberately.
- Roman Reigns of The Shield had this for his two other teammates, Seth Rollins and Dean Ambrose. Even after Rollins stabbed him and Ambrose in the back, he still has this for Ambrose and it's been implied that he still has somewhat of a soft spot for Rollins. In fact, some of the main reasons why he hates The Authority, particularly Triple H, is because of the pure hell they've put Dean through and their corruption of Seth.
- Bella Twins:
- When Stephanie McMahon was using her power to bully Nikki in revenge for Brie embarrassing her, Brie arrived at the show as a fannote and got her revenge - when Stephanie slapped her at ringside, Brie got the police involved and Steph was arrested for assault.
- Even one time when the twins themselves were feuding, Brie prevented A.J. Lee from attacking Nikki at ringside.
- Averted one time when Brie abandoned Nikki to get attacked by Kharma. Nikki forgave Brie, admitting she would have done the exact same thing.
- Undertaker is possibly the best example of this with Kane, whom he's come to the rescue of numerous times. Seeps into Knight Templar Big Brother depending on how far he's willing to go for retribution should be he unsuccessful in saving Kane at the time.
- Stevie Ray is also a real life example. During his podcast, Stand Up For Greatness, he revealed that when he heard that Mike Awesome had hurt his younger brother Booker T in a match, he confronted him. Stevie made it very clear that if Awesome had admitted that he intentionally hurt Booker, he would've kicked his ass.
- Angelica Schuyler from Hamilton absolutely loses it when she finds out that her sister Eliza's husband Hamilton cheated on her and broke her heart. In a cut song called "Congratulations," she drags Hamilton for a whole two minutes. Despite showing some love to Hamilton, Angelica Schuyler will not take any of Hamilton's shit and will always put Eliza, her only remaining baby sister, before anything and anyone in the world. One of her lines in "The Reynolds Pamphlet" sums it up:
Angelica: I love my sister more than anything in this life / I will choose her happiness over mine every time!
- Emma Borden from Lizzie is fiercely protective towards her younger sister. With their mother's death and abusive father, Emma essentially became a parental figure to Lizzie. It is clear she feels responsible for her happiness and well-being (more accurately, lack thereof). Given the subject matter, her efforts to save her sister don't exactly work.
- Inverted in Les Misérables; in some productions Gavorche has this attitude towards Eponine who is his older sister. He tries to go to her when she gets shot, and cries after she passes.
- Broken Saints: Heartwarming Orphan Shandala, the heroine, is looked after by her adopted parents' son as if she were his own flesh and blood.
- DSBT InsaniT: It may seem like Killer doesn't care for his twin sister, but he'll defend her if he finds the need to.
- Etra-chan saw it!:
- Tokusa, after finding out that his fiancee Akane previously bullied his little sister Tsutsuji and planned on bullying her into being her slave once they're married, breaks up with her on the spot.
- Hiiragi, after finding out that she falsely accused his little sister Tsutsuji of stealing money from the wallet she left behind at the restaurant his sister worked worked, vowed to make Azami pay. He eventually uses his skills as a detective to expose Azami for stealing money from her company, leading to her arrest.
- Yuri stands up for her younger sister Tsutsuji when her mother-in-law Azami talked crap about her tea tasting as bad as Yuri's. And she did this while suffering morning sickness from being pregnant.
- Red vs. Blue:
- Agent North, with such a trustworthy, older-brother instinct that he gets assigned Theta, the AI fragment composed of Alpha's innocence. He's the only one in the program qualified to take care of a being of pure innocence. But seriously, watch seasons 9 and 10. The dude lives and breathes trust.
- Blue team. Sure, Caboose is an idiot. But he's their idiot.
- Wash eventually develops it for the Reds and Blues. In the flashback, he tried to mimic North's, but CT wasn't buying it. As of Season 11, Wash especially has this in regards to Caboose, trying to keep him from getting overcome with depression after Church leaves and, in Season 12, making sure Freckles is okay after getting captured by the Feds.
- Gender-flipped with Yang in RWBY, as one of her primary character traits is how protective she is towards her little sister Ruby (to the point that there's an entire Image Song devoted to the subject). When the events of Volume 3 leave her a broken mess suffering from PTSD and depression for several months, it's the realization that their father can't take care of her and go after Ruby at the same time that ultimately gives her the motivation to come to terms with her trauma and begin her recovery.
- Parodied in RWBY Chibi. When Ruby can't open a jar of pickles, Sun tries to help her but also fails. Yang turns up because her "big sister sense" is tingling. She insists on opening the jar of pickles for a reluctant Ruby, who drags Sun off to a safe distance while Yang wages open warfare, complete with using weapons, on the jar.
Yang: You dared to make my baby sister hungry? You're going down jar!
- Dead West has a character, The Porcelain Doctor who is enforcing this trope into others. His aura, called the Devil's Veil, shows him fragile, weak and casts a glamour to make others like him (or wanting to crush him). He cannot turn it off, which really annoys him, but is notable that Gervas, our narrator would feel the same without falling to his aura. His real big brother is more along the lines of a Knight Templar Big Brother.
- Decembersville: Nutty Jimmy is like this to his sister, Sarah. As illustrated here.
- Dimension Heroes: Rob displays this trope towards his fellow Dimensional Guardians, reasoning that as the leader, he's responsible for their safety.
- The Other Kind Of Roommate has Terry showing this for Beth. It's treated as cute and protective before it gets controlling and is explained as the reason they broke up.
- In Worm, this is a major part of Grue's makeup –- in fact, his trigger event came when he was defending his little sister from his mother's abusive boyfriend.
- The Adventure Zone gives us Lup. Even while trapped in the umbrastaff, she still kills off Edward for basically torturing her twin brother.
- PopCross Studios: During Elsa's segment in What If Disney Characters Were In Mortal Kombat?, it's stated that, much like in canon, she did her best to keep her ice powers hidden from the world. Then she learns that Hans tried kill her younger sister Anna in hopes of stealing her wealth. Elsa then wastes no time in hunting down and publicly murdering Hans, not caring that she would end up becoming a wanting criminal for the act.
- Aaron has two brothers getting into a blazing row. When the younger one comes home drunk, the older immediately comes to his rescue.
- The web documentary F*** Kayfabe: Wrestling with Labels -– Cianan and Matt have this towards Bobby. Bobby jokes that himself and Pete both think of the other as their little brother so they decided to be twins instead. Bobby has this for Kev and even refers to him as "my little brother" in the Special Thanks at the end.
- In the Noob webseries and comics, any enemy bothering Gaea has a decent chance of having Golgotha show up for a serious beating, regardless of her location when the bothering started. A variation in that Gaea is a Dirty Coward rather than weak and Golgotha's protectiveness is simply stemming from their history as Childhood Friends.
- The Nostalgia Critic:
- Rob Walker will be the first to admit anything that didn't work out in an episode, but if you go too far and insult Doug instead, he will make you sorry.
- In character terms, while The Other Guy is a Big Brother Bully when the Critic is alive/sober/lucid, he'll always show how much he cares when nobody else is around.
- They're not related but in Demo Reel, after Donnie (Doug) gets kidnapped, badass stoic Carl (Rob) tears down the forest looking after him and makes sure he's okay when he returns.