Say you’re a villain. You’re up against a team of heroes and you need a sure-fire way to rattle them. What do you do? Well, the best way to get everybody worked up is to threaten that character, the one who the whole team feels they need to protect. Congratulations, you’ve just found Everyone's Baby Sister (or brother).
When a Big Bad sets this character in their sights, it immediately becomes personal for every other character involved. Everyone's Baby Sister is most common in works where some of the characters do jobs that regularly put them in harm’s way and that exacerbate their protective instincts, and some of the characters work in safer environments where they’re not supposed to be at risk of physical harm. A team composed of law enforcement officials and scientists or analysts is common here. This character will typically be a younger character who is not usually in any sort of physical danger and, thus, may appear physically smaller and weaker than the other characters. Other characters will feel protective of this character because of a combination of their older age, superior physical strength, and sometimes their occupational responsibility. When something happens to a character and even the Jerkass is unsettled by it, you have a winner.
Sometimes these characters inspire this reaction because they’re impossible to dislike. Other times, the character really isn’t very likable at all; it's just that the other characters feel responsible for him or her. Because main characters tend to be the Action Heroes, this character is more often a supporting character than a lead. By coincidence or design, these characters are often Adorkable Teen Genius types (or early to mid twenties genius, as the case may be), though this is certainly not a prerequisite. These characters also tend to be fan favorites, though this, again, is by no means, necessary. It is not unheard of for The Scrappy to fall under this trope. What’s important here is the reaction of the characters, not of the audience.
Note that this character does not actually have to be anyone’s sister, and a character who is someone’s sister is not guaranteed to fall under this trope.
- Ghost in the Shell: Stand Alone Complex has Togusa for Section 9. Partly because he's the team's only non-Cyborg member and partly because he has a family. This can be seen best in the episode "One Angry Man — TRIAL", where trying to drag Togusa's name through the mud leads a Domestic Abuser and his lawyer to suffer an unfortunate car accident.
- Casca in Berserk. When she's surrounded by enemies, the rest of the Band of the Hawk shows up to rescue her citing the fact that she's the only girl (even though she's the team's second-in-command with only two people able to beat her in combat). Later on, Schierke fits this role (like when Guts starts a Bar Brawl when a drunk gets too close to her), as Casca has regressed to a childlike state due to the events of the Eclipse.
- Alvis in Last Exile is the Living MacGuffin whom most of the series revolves around the other characters keeping away from the Guild. She's also a cute little girl that does cute things cutely, so she tends to inspire everyone to be protective and caring towards her beyond that.
- Hikari in Digimon Adventure is forced into this position by virtue of being the villain's primary target for her entire introductory arc, well besides being The Leader Taichi's actual baby sister. Afterward, her status as the new kid, Mysterious Waif tendencies, suffering a relapse of the illness that prevented her from main character status in the first place, and then trying not to let on because of her selflessness cement it.
- Wendy in Fairy Tail is among the youngest members of the guild, and, being 12 at the start of the series, is several years younger than most of the current generation, who are roughly 16-18. As a result, she ends up serving this role to the others, particularly the "strongest team" of Natsu, Lucy, Gray and Erza - she says she considers her fellow Dragon Slayer Natsu to be like an older brother and in supplemental materials, the others are said to regard her as a younger sister.
- Haruhi from Ouran High School Host Club can be this to the members of the host club. While some of them actually have crushes on her and act out of jealousy, Haruhi is still a first-year (one of the youngest members) and all the guys are willing to go above and beyond to protect her at even the slightest inkling that she's in danger.
- Bonnie from the XY saga of Pokémon is Clemont's little sister, and his Big Brother Instinct is combined with similar protective feeling from Ash, Serena, and their Pokémon.
- Aviva Weinberg of Relative Heroes is literally the baby sister of her team as it's made up of her siblings. When the Weinbergs help with the invasion of Zandia several members of Young Justice and their allies make sure to keep her out of the line of fire or at least behind forcefields and bulletproof individuals since she is incredibly young for a superhero, even when compared to the rest of the group which is almost entirely made up of teenagers.
- In Runaways, Molly and Klara fill this role, to varying degrees - Molly is everyone's baby sister, but Klara is more of a little sister to Nico and Karolina, while Chase and Victor treat her more like a Ladette. Oddly enough, the baby sister aura even seems to affect other people in the Marvel universe - for instance, during Secret Invasion, Speed of the Young Avengers literally went out of his way to keep Molly and Klara safe during a battle with the Skrulls.
- Secret Avengers Vol 2 seemed to play Mockingbird like this during the "How to Maim a Mockingbird" arc. Hawkeye, her ex husband, was visibly torn up from seeing what happened to her during the arc, Black Widow stopped joking and talked to her the way a worried sister would when she realized the seriousness of the situation, and War Machine actually quit the team in protest and disgust at the fact Maria Hill allowed Mockingbird to get into that situation (and is joined with Hawkeye in this), while Maria Hill herself is clearly not happy with herself for allowing this to happen and makes a promise to find her. Even Taskmaster, the Token Evil Teammate, is pretty shocked to see how broken she is and is clearly concerned for her welfare.
- Cheedo the Fragile in Mad Max: Fury Road is the youngest of Immortan Joe's unwilling Wives and, as of the time of the film, the only one who hasn't already been raped by him due to being too young to breed, so the other Wives are extremely protective of her and try to shelter her from Joe's true monstrosity, which results in Stockholm Syndrome where Cheedo at one point attempts to turn herself into Joe while on the run. The prequel comic also shows that The Dag went so far as to directly interfere when Joe tried to rape Cheedo, and was savagely beaten for it, causing Cheedo to blame herself.
- On NCIS, the best way to make something personal for the entire team is to threaten Abby Sciuto. This can be seen when you compare an episode like "Bloodbath", where someone is after Abby, to episodes like "Frame Up", "Reveille", "Probie", and "Recoil", where the threats against Tony, Kate, McGee, and Ziva are taken seriously, but not as personally.
- Newcomer Ellie Bishop appears to be falling into this role too; everyone has taken her under their wing to some extent, and they've all expressed a desire to beat the tar out of her adulterous husband. When her three actual older brothers show up, they take the trope to its logical extreme.
- Firefly: Most of the time, when a crew member gets injured, it gets more or less shrugged off. When Kaylee gets shot in the pilot, it's a different story. Mal threatens to toss Simon into space if he fails to save her. Jayne, who rarely shows concern for anyone, immediately goes after the man who pulled the trigger.
Book: You're not going to kill this man.Jayne: Well, not right away.
- Agents of S.H.I.E.L.D.: Leo Fitz and Jemma Simmons both qualify as this trope. The other characters' reactions to their lives being endangered in "F.Z.Z.T." and "Beginning of the End" exceed what we've learned to expect when Coulson, May, Ward, and Skye risk their lives. Fitz and Simmons are the non-field agents on the team, and two of the youngest. In "F.Z.Z.T.":
May (of Simmons): She's only a kid.
- On Once Upon a Time, Henry is the only character who could have gotten Emma, Snow, Charming, Regina, Rumplestiltskin, and Hook on a ship to Neverland together. This may be a factor of his age and the fact that he's related to almost the entire main cast.
- Law & Order:
- This tends to happen with the Assistant District Attorneys. We've only really seen it played out with Claire Kincaid and Alexandra Borgia on the original Law & Order and Alex Cabot on Law & Order: SVU, but it did play out with all three of them.
- On Law & Order: UK with junior CP Alesha Phillips. The reactions from the rest of the team when she's raped make it obvious that harming her is a Berserk Button for them.
- Arrow: Despite Ollie having a genuine sister, this role goes to Felicity Smoak instead. She's the only non-combatant on The Team, and Ollie and Diggle go to great lengths to ensure her safety and keep her hands clean.
- On a meta level, Willow from Buffy the Vampire Slayer - the writers quickly discovered that threatening her was one of the best ways to get the audience to react. An aphorism among the writing staff was "When in doubt, put Willow in danger".
- On Guiding Light, Abby (who was already this to the townspeople) was attacked and nearly raped by a gang member. When the rest of the gang began making threats to her in the hopes of preventing her from testifying, the Token Good Teammate (a) reminded them of what he'd done, clearly trying to invoke Even Evil Has Standards, and (b) informed them that, "You know who she looks like? Tony's (gang member's) sister. Might as well have been her he tried to mess with."
- Dark Matter: Five, as the only non-criminal member of the crew, tends to be kept on the ship for her own safety most of the time, despite the fact that her technical expertise is extremely useful on the rare occasion she ends up on a mission.
- On General Hospital, local Mary Sue Robin was this to everyone in town. The trope was so deeply in place that when Carly came to town, the first indication that she was going to be a villainous-type was that not only did she dislike Robin, she couldn't understand why everyone else adored her.
- Community: As the youngest member of the study group, Annie Edison gets treated this way a lot, especially in early seasons (in later seasons she mostly proves that she's more mature than the rest of them). This causes friction with her primary love interest, Jeff, who is sixteen years older than her. Early on, he repeatedly uses her age as an excuse not to date her. In later seasons that excuse mostly disappears, and Jeff's self-loathing is the main thing that keeps them apart.
- Persona 4: Nanako Dojima, protagonist Yu Narukami's little cousin. She instantly becomes his team's little cousin as well, and they all become very protective of her. When she's almost killed near the end of the game, the entire team goes into Roaring Rampage of Revenge mode.
- In Persona 5, Futaba appears to play a similar role, despite being older than Nanako (she's two years younger than Makoto and Haru, and one year younger than everyone else in the party). After saving Futaba from committing suicide by destroying the manifestation of her guilt over her mother's death, the rest of the Phantom Thieves bond with her, helping her to come out and see others, and eventually take her to the beach with them. During one randomly occurring exchange while exploring Mementos, Makoto points out that seeing someone as cute as Futaba reminds her of the time when she wanted a younger sister.
- Merrill from Dragon Age II is this to most of the party. Only Fenris and Anders dislike her due to her use of Blood Magic.
- In a non-mage-Hawke playthrough, Bethany - who is Hawke's actual little sister - also qualifies whenever she's present. Everyone in the party likes her, including the staunchly anti-mage Fenris; Anders is lukewarm at worst. If she goes to the Circle instead of dying or joining the Grey Wardens, this trope remains in effect even after Act 1.
- Sheva Alomar of Resident Evil 5 is described as the "little sister" of her BSAA squad by her C.O. and mentor, being the youngest and greenest member. Of course, Chris Redfield is told of this long after she's ably demonstrated her own competence and badassery, and his survival depends on her throughout the mission as much as hers depends on him.
- Sachi from Rara Magi, One of the youngest members of Music Club is doted◊ by almost every older members in the club. Combined with her begin a sweet nice girl and innocent to the core, it's no wonder why everyone adored her. It's become a plot point when she starts to act out of character due to Noise interfered during later chapters.
- In Darths & Droids, Ben's little sister Sally is initially brought to the group because he's forced to babysit her, but she quickly gets integrated into the game as Jar Jar Binks (and single-handedly makes the character enjoyable.) In the early strips when she was young, the GM and players tended to be protective of her, cheering her on and giving her extra chances at failed dice rolls.
- This is part and parcel of Piffany's Incorruptible Pure Pureness in Nodwick. For all that Artax and Yeager engage in Comedic Sociopathy on many others, they will not tolerate anyone being mean to her. Their Let's Get Dangerous! cry is "He/She/They made Piffany cry!"
- In Errant Story, Misa is this, very much against her will: she desperately wants to go out and have adventures like her idol Sarine, but because she's the youngest of all the elves and is considered to be of near-miraculous origins, the elves won't let her, even during their final, species-threatening battle. Finally averted, however, when at a key stage of the battle, Sarine herself starts treating her like a grown-up.
- SCP Foundation: SCP-085, "Cassie," is a living 2D drawing of a young woman who thought she was living a reasonably productive life in a post-apocalyptic wasteland until she found out the truth behind her situation. The Foundation as a whole feels pretty bad about everything that's happened to her, partly because she only exists because they were playing around with some other SCPs. A lot of effort is made to make her feel comfortable, from finding things she can use in her 2D state to discussing getting her a psychiatrist.
- In Red vs. Blue, Michael J. Caboose serves this role over time. Even when the "opposing" teams are insulting one another, they make time to be nice to him, and actually make an effort to make sure he feels good about himself. The times when he is upset by an outside party are actually treated as deadly serious and horrible things to do, especially by Tucker.
- Invoked by Hun in the 2003 Teenage Mutant Ninja Turtles cartoon, in which April is a mix of little sister and big sister to the turtles (she even acts as a motherly figure sometimes):
Hun: Those green freaks feel great affection for you. Losing you should take the fight out of them.
- A 'baby brother' example, in Sonic Sat Am, Tails is one of the main targets for Robotnik to grab, mostly due to a) his age, and b), his little brother relationship to Sonic. (If it's not Tails, Sally is who Robotnik will nab, being Sonic's love interest and all.)
- Steven from Steven Universe. The Crystal Gems are his Parental Substitutes since his mom, their former leader, suffered the supernatural equivalent of Death by Childbirth. His personality also gets him "little brother" status with every teenager in Beach City, except Lars (not that this dissuades Steven from trying). As of Season 5, even Lars has warmed up to Steven.
- Padparadscha seems to be this for the off colors. Lars, a self-admitted Dirty Coward even takes on the robloids to protect her and Rhodonite is quick to guard Padparadscha as well. Because of her delayed reactions, Padparadscha is not always aware she was in danger until after her comrades have helped her.