The Little Sister Heroine is a trope common in — usually but not always Japanese — media that combines the idea that little sisters are cute and that cute things are to be fetishized. It goes like this:
Little sisters are cute, right? And cute girls draw in viewers. And cute can be sexy. Take a few more logical steps down the line, and suddenly little sisters are love interests in anime, video games, and visual novels. It is in fact one of the most popular types of heroine or love interest in Visual Novels, branching out from there into anime, perhaps surpassed only by the ever-present tsundere and childhood friend. Like the similarly ubiquitous Childhood Friend Romance, it is extremely prone to lampshade hangings.
As a Love Interest trope based on the little sister/older brother dynamic, the story will usually dwell on the romantic tension between them. Despite the name, Little Sister Heroine attempts to mitigate the obvious incestuous fallout; they're usually either a step-sister, adopted or, more rarely, technically a cousin or distant relation. In almost all cases, the younger sister is going to be either some degree of tsundere, childish and clingy, or simply highly devoted to their older brother. Finally, the relationship between the two will likely be noticed or exaggerated by the rest of the cast. If you don't notice at least several of these general characteristics, it's probably not this trope.
If the relationship remains incestuous, the sister usually completely takes the initiative, in order to allow the author to save the main character from the moral responsibility and use thematic fanservice in the work. Thus, in the majority of plots concentrating on the incestuous relationship with the little sister, the sister takes an active role throughout the story in seducing her brother, while "protecting" him from all the other girls. If we are talking about siblings with little difference in age or Half-Identical Twins, then her childish traits tend to be less pronounced, tending toward greater borrowing from Childhood Friend Romance tropes.
In this regard, it should also be noted that—similar to other such plots—the main character and his sister will in most cases be represented as a kind of duet, as a result of which the other cast members will mistakenly take them for twins, even in the absence of kinship or the presence of a difference in age. However, this trope works only if the little sister-ness is central in this work.
It may include elements of him needing to protect her, a fear that someone else might hurt her or break her heart, or that other guys just aren't good enough for her. It is also related to the idea that this sibling is theirs, and they don't want someone else to "steal" them. In this regard, a great many little sister heroines feel aversion to all other men than their brother, in order to emphasize their devotion to him. In other cases, it may also mean that an outsider guy can become a couple with the little sister only if he can be equal to or better than her brother, which is often used for harmless resolution romantic tension between brother and sister.
It rarely works like this in real life. Those who have younger sisters themselves and hear about this trope often claim that they're annoying rather than cute, making this trope more wish fulfillment than anything like an accurate portrayal of sibling dynamics. Thanks to the Westermarck Effect, it is far more likely that the idea of any kind of romance with your sister would be either laughable or even outright disgusting.
Also known in some circles as an imouto heroine. Accordingly, the works having an imouto as main romantic interest, marked as Imouto-titles.
Note that BrotherSister Incest is not exclusive to Asian media, but that when it is portrayed in Western media it tends to be between twins or the woman will be the older sibling. Western media sometimes doesn't bother to minimize the Incest Subtext, though if it does the excuse will usually be that one of the siblings is not actually human (or neither of them is). If it's framed more from the point of view of the younger sister who likes the older brother, it's Big Brother Attraction, although both can occur simultaneously. See also BrotherSister Incest, Flirty Stepsiblings, Big Brother Mentor, Big Brother Instinct, and sometimes Not Blood Related.
- Invoked in Blend-S, where Mafuyu is hired by Dino to play this archetype in Café Stile; when waiting tables, she acts like a cute, cheerful little girl who addresses customers as "Onii-chan". However, she's actually a 20-year-old college student who's older than she looks, and her real personality is that of a Deadpan Snarker who can be brutally honest.
- Something of this is between Lelouch and Nunnally in Code Geass. Nunnally hits a lot of the check points — young, cute, Damsel in Distress, adores her older brother who is very protective of her (he just started a damn war to protect her that's all) — but isn't certainly his love interest, falling under the Not Love Interest in importance. That said, they have Incest Subtext moments and a lot of extra materials lampshade how ambiguous their relationship is due to their devotion to each other. Played for Laughs as a general rule, but it's worthwhile noting they are royalty and royalty has a long tradition of incest. Extra materials also reveal their own mother wanted to play this straight between them. Ick.
- Played slightly more straight between Lelouch and his half-sister Euphemia. Lelouch fully admits she was the first woman he loved and when she was younger Euphie wanted to marry him, not to mention in the present still shows signs of attraction to him. Euphie has an entirely different love interest in Suzaku however and Lelouch is forced to Mercy Kill her after a horrible mistake.
- A Couple of Cuckoos has Sachi Umino, protagonist Nagi's little sister. She didn't start seeing him as a man until she learned that they're Not Blood Siblings after all since Nagi is adopted, but Nagi himself has trouble not seeing her as a little sister.
- Parodied with the 4koma manga Cyclops Shoujo Saipu which plays everything straight... except the "little" sister is a 180 cm (5 ft 11 inches) tall middle schooler with Gag Boobs, and the older brother is one head shorter at 165 cm (5 ft 5 inches).
- Genshiken's club of otaku, very aware of these tropes, raises the possibility when they find out newcomer Kanji has a younger sister. It is then shot down when he says that only those without younger siblings could fetishize them so.
- Nao from I Dont Like You at All, Big Brother!!, whose life goal is to hook up with her adopted brother Shuusuke.
- Chino in Is the Order a Rabbit? is being seen by many of the cast as one, to her annoyance.
- Koi Kaze explores how something like this might actually happen in real life. The brother and sister in the story haven't seen each other in over ten years - when the sister was just a toddler - and as such don't recognize each other at first. The brother, Koshiro, near instantly falls in love (causing him a great deal of distress when he discovers their blood relationship) while the sister, Nanoka, at first is doting on her newly reconnected older brother, only to later discover that her feelings have deepened into romantic love. Koshiro's colleague Chidori notes the odd atmosphere surrounding the two when Nanoka visits Koshiro at his job.
- Implied/parodied in The Melancholy of Haruhi-chan Suzumiya where in the final episode Yuki makes a romantic Visual Novel with Kyon as the player character and with 10 heroines, one of which is Kyon's little sister.
- Inverted in Onee-chan ga Kita where the main heroine is the protagonist's older step-sister.
- Repeatedly commented on and averted in Shinozaki-san Ki wo Ota Shika ni!. Akina thinks her brother Makoto is disgusting. Makoto, for his part, has no interest in Akina and is seriously weirded out by how so many people keep bringing this trope up.
- Sket Dance have Saya, a redheaded tsundere with large breasts and twintails. Bossun and Switch remarked that she looks exactly like a Visual Novel character, and is only missing the "little sister element". Turns out she is actually the younger sister of another recurring character, i.e. student council president Agata.
- The World God Only Knows has Keima absolutely refuse to acknowledge that Elsie is a proper little sister because she doesn't match the template from his beloved gal games. Eventually, however, he decides to acknowledge her, but only because he wants to get their contract completed quickly. As time goes on, she eventually really does become his little sister, moving away from this trope in the process.
- Yosuga no Sora has Sora, the main character's twin little sister. She has very strong feelings for her brother, going as far as thinking of him during... certain private times.
- A justification is attempted by showing, in a flashback, that Sora spent most of her childhood hospitalized, and so Sora and Haruka didn't spend enough time together for the Westermarck effect to kick in. Why Haruka was kept away from his sister for so long is never explained.
- Viserys Targaryen in A Song of Ice and Fire has raised and protected his beautiful little sister all her life. By their family's traditions, he should've married her, had circumstances been better. When it's time to marry her off to someone else, even though he decides to do it, he barely goes through with it. The little sister in question is reasonably aware of all this, and not cool with it.
- Taken to a whole new (hilarious) level in Nisemonogatari with Araragi's little sisters, Karen and Tsukihi. That infamous toothbrushing scene.
- Enju Ahiara from Black Bullet is seen as this from Rentaro's perspective. This can be a deconstruction of this trope, as Enju's Precocious Crush moments not only serves as an annoyance to Rentaro, but Enju's high corrosion rate and the fact that she's slowly turning into a Gastrea serves as an emotional burden to him.
- Brothers Conflict has this as its premise, with all of the stepbrothers going after the little sister from the point of view of the "Little Sister" herself. The series also has a Rare Male Example in Wataru, the Token Mini-Moe and Cutie.
- In Date A Live, Kotori Itsuka is a parody of this when first introduced. Her foster brother Shido (and others) even call her his "cute little sister". It then gets subverted when we meet her other personality. The romantic subtext is unclear. She is implied to have some not-quite-sisterly feelings for her brother but doesn't have a problem setting him up with the Girl of the Week (It Makes Sense in Context). The only time she goes into Clingy Jealous Girl mode is when she meets his biological sister, Mana, at which point she reveals the level of her interest in him by arguing about who's better and finally declaring that not having any real relation is better because a real sister couldn't ever marry him.
- Parodied in Dokkoida?!, where the main character lives with an alien girl masquerading as his little sister. Thanks to a soap opera episode following this trope, she and the majority of their neighbors (who are also aliens) become convinced that it applies to the main character as well, thanks in part to Dirty Old Man Dr. Marronflower using his own library of H-games as evidence. Hilarity Ensues when the two who have a crush on the hero attempt to invoke it to their advantage, leaving him completely confused.
- The Garden of Sinners has Kokutou's little sister Azaka. She has an inherent interest and draw towards things that are taboo, so she develops an incestuous crush on her brother. Azaka is the template for Akiha in Tsukihime.
- Miyuki Shiba from The Irregular at Magic High School can be seen as an example of the collection of all the clichés of such heroines and as well as an example of a rather original attempt to play with them. On the one hand, she idolizes her brother and declares willingness to do whatever he wants with her. On the other hand, her "silly little sister" behavior is only a mask to deceive her brother and convince him of the need to continue to take care of her. Later, it turns out that as she was artificially created and genetically improved, she can have a healthy family with her brother while continuing to be his biological sister and having the same parents as him.
- Kureha of Mayo Chiki! is a member of Jirou's Unwanted and Supporting Harem. Observe her reaction upon finding that Jirou, who has gynophobianote may have a girl over at the house. It's not ignoring something that's none of her business, or happiness that her brother has finally gotten over the condition that has crippled his social life, but rather to kick down doors and scream:
Kureha: Who's been swindling my brother!?
- The entire premise of My Sister, My Writer is themed around this trope; the main heroine, Suzuka is in love with her older brother Yuu, and the light novel she writes that becomes an overnight success is about this kind of heroine. The trope itself is discussed more than once.
- The premise of Oreimo is largely based around this trope. Kirino plays games with this trope as their focus and is the tsundere type herself. The relationship between her and her older brother Kyousuke is probably the biggest focus of the series.
- An interesting case shows up in A Sister's All You Need. The main character's sister isn't one, but his father assumes that he's a significant threat to her because of the MC's incredibly unhealthy obsession with younger sisters in fiction. As such, he forces her to present as male around her brother out of fear of what he'd do to her if he knew she's actually his sister. Furthermore, when he does eventually find out that his sister is a girl, he goes into a funk so wide he takes a hiatus from writing; since he refuses to see his sister like an imouto heroine, he recontextualized his obsession and found the whole practice gross. However, there is a younger sister that invokes the trope, but she is constantly rebuffed until she threatens her brother with her own suicide if he doesn't stay by her forever.
- Suguha/Leafa in Sword Art Online is actually Kirito's cousin, but otherwise fits the trope to a T, as they were raised together as siblings by her parents and she harbors a Big Brother Attraction towards him. She grows out of it by the end of the Fairy Dance arc, though.
- Ar tonelico II: Melody of Metafalica: Cocona is Croix's adoptive sister and very cute. She confides in Cloche that she is in love with him but his only love is as a brother because of their age difference, though if you don't secure any of the other three endings you default to her. The sequel suggests that this is the true ending, however.
- Kind of a Zig-Zagged Trope with Noel Vermilion because she's not actually Ragna's little sister but rather a clone of her. Everything else about the trope is played straight. It's heavily implied she has a crush on him, and even if she doesn't, she looks up to him like an actual older brother and the normally withdrawn and antisocial Ragna is fiercely protective of her. Their relationship is a major focus of the series at that.
- In a more explicit and much darker example of this trope, there is v-13, another clone of Ragna's little sister who, as a Murakumo Unit, serves as one half of Ragna's titular, incomplete Artifact of Doom. Said Artifact actively tries to become whole, with the Murakumo Unit's programming compelling v-13 to get as close to Ragna as possible. This appears to be interpreted by v-13 as "attempt to get into Ragna's pants" judging by how most of her dialogue with him is peppered with Double Entendres and innuendos and their fights are showcases of Interplay of Sex and Violence. Ragna is not thrilled... A lot of the plot revolves around Ragna and v-13's relationship, Ragna's attempts at saving v-13, her inability to comprehend and appreciate their relationship beyond anything but this Masochism Tango they've got going on, and both's absolute hatred of the world and what they choose to do with that hatred.
- Then there's Hades Izanami, a goddess of death who uses the body of Saya, Ragna's actual little sister, as a medium. Though Izanami herself has scant interest in Ragna, she reveals that Saya's love for him is infectious and pushes her to reach out to him. This largely comes off as Izanami coming on to Ragna really, really strongly. It's not entirely clear if Izanami's behaviour is an accurate representation of Saya's feelings for her "dear onii-sama", or if Izanami is twisting Saya's feelings into something obscene just to mess with Ragna, or if human emotions are just so alien to Izanami that she genuinely doesn't know how to express love without sexualizing it.
- Complicating matters is the fact that both Noel and Izanami/Saya contain two halves of The Origin's soul. The Origin is a young Murakumo Unit from a previous universe who was installed into the Master Unit: Amaterasu. After destroying her old universe and dreaming a new one into existence, The Origin created a "central fiction", a lynchpin that the whole universe revolves around, in much the same way a work of fiction revolves around its central main protagonist. Said "central fiction" was Ragna who The Origin dreamed into existence as a Big Brother/Knight in Shining Armour figure meant to free her from her imprisonment within the Amaterasu Unit.
- Dark Souls plays with this trope in a very dark way: after slaying the evil Chaos Witch Quelaag in her lair, the Chosen Undead discovers that she was taking care of her blind, crippled little sister, referred only as "the Fair Lady" by their servants. Since the Fair Lady is blind and the Chosen Undead is a Heroic Mime, she mistakes you for her sister (whom she adores), and you can then join their Covenant, whose sole purpose is to procure means to (slightly) ease her suffering, which is what Quelaag was trying to do when you murdered her. While there is no romantic subplot (mainly because the Fair Lady is in constant pain and is also a giant spider from her waist down), there is most definitely a Lady and Knight dynamic going on.
- Parodied with Desco◊ in Disgaea 4: A Promise Unforgotten. Desco was created specifically to fulfill Fuka's childhood wish for a little sister, and as such was implanted with all of the classic traits of a Little Sister Heroine. However, since Fuka is an eccentric individual who wished for her Little Sister Heroine to come with "advanced functioning to help me take over the world," Desco was also created with Lovecraftian Superpowers and a compulsion for complete annihilation. The result is the world's most Adorable Abomination.
- Disgaea Dimension 2 gives us a straighter example with Sicily, Laharl's Long Lost Sibling. While she naturally plays the trope straight, she tries to really invoke it for all it's worth by playing up her cuteness and offering sweets to convince Laharl to give up the title of Overlord. It only serves to further piss him off.
- A non-Japanese example from a Western developer that likes playing with JRPG tropes: Bethany Hawke from Dragon Age II hits almost every note of this trope. Young? Check. Cute? Check. Needs rescuing from time to time? Check. Adores her older sibling? Double check! Romanceable? Nope.
- Mia in Duel Savior Destiny is definitely the Clingy Jealous Girl type with some level of tsuntsun going on. Her actual degree of relation to Taiga gets retconned, though. In the epilogue of her route, it's hastily claimed that it sure is convenient that they're not related. In the final route, it turns out they actually do share a blood relation and are probably half-siblings.
- In Fire Emblem Fates, Princess Sakura of Hoshido and Princess Elise of Nohr follow the archetype. Each girl is the youngest of the local Royal Families (Sakura is the Avatar's blood-related sister (only not) while Elise is the adoptive sister kind), both adore the Avatar in her own ways, and both will be heartbroken if the Avatar doesn't join the side she supports.
- Another non-Japanese example: Luxanna Crownguard AKA Lux from League of Legends. She is the little sister of Garen, a distant one, and a Cute Witch that serves the nation of Demacia well, just like her big bro. And while she certainly has some things to call out on him, she clearly respects him a lot. She's in about her teens, and so far she seems to be hooking up with another champion, Ezreal. But that doesn't stop Fanon to think that Garen may not want to hand his sister over that easily to Ezreal.
- Men at Work 2 has Mena as a possible love interest, though not the main one.
- Persona 4 has The Protagonist's cousin Nanako Doujima, who refers to him as "Big Bro" and gains a Precocious Crush on him at the end of her social link. Nothing gets made of it, unless the player also maxes out her father's social link, in which case his only protest is that Nanako is still too young.
- Yandere Simulator is an Animesque game that uses a ton of Harem Anime tropes. Therefore, it's not surprising that there's a rival that happens to be your Senpai's younger sister. However, she's not actually interested in them that way; she's clingy, and at the end of her week she'll convince them to not have a girlfriend for her sake. She's no less a threat for our Yandere Villain Protagonist, though, so she'll have to be dealt with.
- Aselia the Eternal - The Spirit of Eternity Sword has Kaori, but in the end, she turns out to be more of a Living MacGuffin than a heroine and does not even have a proper ending. Yuuto is frequently accused of being a siscon due to his overprotective nature, but in the end, it turns out to work in reverse due to her unrequited affections.
- Canvas 2 has Ellis, though she's actually his cousin. She tends to be extremely clingy and childish.
- Chaos;Head has Takumi and his little sister, Nanami. The way the game works means that as with the other female main characters, you can trigger certain 'delusion' events about her that occur only in Takumi's head, but that he doesn't control. Or something. Though it turns out that he's not Takumi, exactly. It's complicated.
- Inverted in Crescendo (JP), where the older sister (Ayame Sasaki) is the one who gets the path. And yes, she's very pretty too.
- Nemu in Da Capo, where she's even the main heroine. In a rare case of not being completely Oblivious to Love, Junichi is in fact perfectly aware of how she feels but isn't quite comfortable with a relationship upgrade. Nemu is quite the tsundere and very clingy.
- Da Capo II actually has two adoptive sister figures, but the one who plays it up more is Yume, who is younger than Yoshiyuki. Yume, a clear Nemu expy, is in general less clingy. Everyone at their school seems to be jealous of Yoshiyuki having such a cute little sister despite his frequent claims that there's nothing between them.
- The Devil on G-String has Kanon. Not only is she a heroine, but Kyousuke appears to be the only person she actually trusts. Kyousuke himself lampshades the romantic overtones of the trope but doesn't appear to take it seriously until she confronts him about it halfway through the main storyline, at which point her own miniroute splits off.
- In Family Project, Matsuri eventually moves in to a little sister position for Tsukasa, even adopting the Japanese Sibling Terminology. Her route, however, tends to focus on the fact that she's far younger than he is.
- The H-Game Imouto Paradise and its two sequels have a Themed Harem of no less than five little sisters, all of them different but adorable all the same.
- Kana: Little Sister has the main character's terminally ill little sister as the main heroine. The VN itself is possibly the Trope Codifier for this kind of relationship.
- Kara no Shoujo has Yukari, but despite the BrotherSister Incest all over the story, you cannot sleep with her. The very idea clearly would not occur to Reiji.
- She does have a topless CG though.
- Fate/hollow ataraxia as a general sequel to Fate/stay night that contains all of its continuity has open acknowledgement of Ilya as Shirous little sister, a relationship set up in several endings. And she really exploits it too: She's clingy to the point of nearly picking up her former yandere traits and tends to act her apparent age rather than her real age. She still gets no adult scenes, though, despite being the closest thing the series has to a fourth heroine. Also, worth pointing out the two aren't actually related. Shirou was adopted. She's also not really his little sister either, she's actually older than him. It's more invoked than anything.
- Akiha in Tsukihime is largely written around this. Unfortunately for her, except for in her own ending this means she never graduates from 'little sister', even in sequels and expanded universe material. A definite tsundere type, she's even called Imouto (Little Sister) by Arcueid rather than her actual name.
- Lampshaded with Rizu in A Profile.
Masayuki: I'm not marrying you. I want to make sure you don't fall for me or anything. We're only stepsiblings, so I thought I should make that clear.
- Suika subverts the general trend of the little sister being a heroine when Akane stabs her stepbrother to death should he choose her. She's actually in love with his fiancée, so choosing her means that Akane feels that it's okay to take his place and brainwashes his fiancée accordingly.
- Yume Miru Kusuri has Aya, but her route was cut. It is strongly implied that she fell in love with Kouhei because he was always subtly out of place in their home, making it difficult for her to see him as her brother.
- Like the The World God Only Knows example above, Kouhei appears to be frustrated with her at one point for not being anything like the little sisters in H-games. "You're supposed to call me onii-san!" Aya is appropriately confused.
- In the closing stages of World War II, Imperial Japan created the Chiran Nadeshiko (Ladies of Chiran) Units. A bunch of middle-school and high school girls served as surrogate "little sisters," providing housekeeping and company for transient pilots at Chiran Airbase in Kagoshima Prefecture, during the brief periods between their arrival from training bases elsewhere and their departure on their fatal missions. The unit was highly publicized at the time and may even be the Trope Codifier... but growing close to so many young men (some being almost the same age as them!) that were sent off to die in "Kamikaze" attacks took its toll on the girls, so the project was ultimately pulled under the "Harmful to Minors" label. Their story can be found in books like BLOSSOMS IN THE WIND: Human Legacies of the Kamikaze, by M.G. Sheftall., or Tokkou no Machi: Chiran by Sanae Satou.