Sometimes, two characters of the opposite sex are very close - so close that you might expect them to be dating. Except they're not. They admit that they matter a great deal to each other, that they would die for each other, that they can't imagine life without each other. They aren't just any pair of friends. They're as close as family. They're like brother and sister!
It's rather sad then, that some people refuse to see the difference between this and a dating relationship. Especially when there may be a distinct age gap between the two characters. The couple is at pains to explain that this isn't the case. They love each other, like a brother and sister love each other, but they're not in love. They really are "just" friends. Other characters will waste no time in pointing out the one tiny but debilitating flaw with this theory: They are not brother and sister.
This doesn't change the fact that their love is completely platonic with no sexual attraction on either side. No matter how other people view them, they can't see each other in that light.
The Greek word for this love is storge, as explained in detail in C. S. Lewis's book specifically on the subject, The Four Loves. The fact that this doesn't stop everybody is a whole other can of worms all together. Relative Error would be somewhat the inverse of, but definitely comparable to, this trope.
Platonic Life Partners is a sub-trope, in which the characters are not only close, but form each others' primary relationship. If you can imagine the characters as members of the same sex and having them as Heterosexual Life-Partners, then they belong under Platonic Life Partners. Of course, almost all instances of Platonic Life Partners fall under this trope, but to avoid redundancy they should not be listed on this page.
Simply being friends doesn't make a pair and example nor does just being True Companions. Because most childhood relationships are inherently platonic, friendships between children do not qualify unless they continue into adulthood and remain platonic. Contrast Just Friends for when this does actually blossom into romance. Not to be confused with Unlucky Childhood Friend in which one side does have feelings for the other.
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Anime & Manga
Bleach: Toshiro Hitsugaya and Momo Hinamori were raised together by the same old woman they refer to as "grandmother", a Rukongai custom where strangers form family groups because it's nearly impossible to find blood relatives.
Tsubaki and Black☆Star from Soul Eater could also count. Tsubaki is pretty much like Black☆Star's older sister; Black☆Star is the irritating little kid Tsubaki adores and detests at the same time.
One could argue that the relationship between Natsu, Gray, and Erza from Fairy Tail amounts to this. Shipping aside, they interact with each other and protect each other with all the gusto of very close siblings, especially Natsu and Erza. When in a bath, she shows no embarrassment in regards to them being included (due to her upbringing and general lack of a nudity taboo regarding the two) and will occasionally invite them to bathe with her, much to the chargin of any other girls in the bath with her.
Mahou Sensei Negima!: Asuna sees Negi as a little brother she must protect. In reality, he's probably actually her nephew. Great-Grandnephew anyway, with the number of greats unknown. Point is, she is almost certainly the sibling of one of his direct ancestors.
The main theme of Gunslinger Girl is the relationship between each team of cyborg girl assassin and their handler, referred to as "fratello" (Italian for "brother"). Also seen in terrorist couple Franco and Flanca, who choose their names in a deliberate invocation of this trope.
Hilshire: We're fratello. We don't need to talk about love.
In 6-OVA Animated Prequel to the Japanese monster movies, Zeriam, known as "Iria: Zeiram the Animation", the title character Iria is shown as an apprentice to her 'older brother' Guren, whom is later believed to be killed by Zeriam, yet hinted at possibly still being alive. During one scene, the orphan Kei asks Fujikuro, "Isn't he her brother, though?" To which, Fujikuro replies, "Yeah, I guess you could say they're like brother and sister." However, the story never really develops further on this comment.
Hungary and Poland in Axis Powers Hetalia are hinted to be this. Makes sense considering the traditional friendship between the countries in Real Life.
Can also apply to Switzerland and Liechtenstein though their relationship is more Not Blood Siblings.
Word of God also describes France's and Monaco's relationship like this, at least from Monaco's side. Confirmed when she calls him nii-san, or big brother, in one comic.
Although it initially appears that there's romantic tension between Erin and Melan in Brigadoon: Marin and Melan, she later reveals that he is like a brother to her.
At first it looked like Bulma and Yamcha from Dragon Ball fame would become an Official Couple, but their relationship amounted to this in the end with surprisingly no bad feelings. Yamcha tried to fight for her affections for a while, and he was a bit bummed out that he lost for a time. In the end it looks as if everybody's happy with how things turned out.
Shipping and Fan-Preferred Couple aside, Bulma and Goku have also been friends since childhood and neither has ever expressed romantic interest in the other in-universe. The closest they ever came was when Bulma saw Goku as an adult for the first time and remarked on how handsome he'd turned out, and even that could be interpreted as sisterly affection rather than genuine romantic interest, especially since she also made the same remark about Trunks later on in the series.
Ban and Himiko from Get Backers. After spending almost 95% of the series like this, the 37th volume reveals that they might not be completely unrelated after all — but the exact nature of their blood relationship is not specified.
This is how Flit feels about Emily in Gundam Age, but he eventually marries her to make her happy after the girl he really wanted to be with is killed in battle.
Winry and Edward are this in the first Fullmetal Alchemist anime. Averted in the manga and in Brotherhood (the second anime); in these, she has this sort of relationship with Alphonse but is obviously Edward's love interest.
Yuzu and Daisuke from Bitter Virgin have been friends for years and have this sort of friendship.
Hulk and She-Hulk, although they are cousins, and were partially raised together.
Wonder Woman and Superman: they've been shipped together for years (both in-universe and out, to a lesser degree), and are frequently paired up in Elseworlds and Alternate Universes, but they've never been anything more than close friends in the main continuity (Though a common Alternate Character Interpretation is that while they do love each other, it's just that Clark loves Lois Lane more, and Diana respects that so they keep their relationship at this level).
Dick Grayson and Donna Troy from the New Teen Titans. They really act like brother and sister and shared leadership of the team. Which is surprising given that he is the Chick Magnet and she looks like a younger version of Wonder Woman. Also, he reallyprefers redheads.
Dwight and Miho from Sin City have this relationship.
Storm and Gambit from X-Men have had this relationship ever since they met in Cairo. Storm got Gambit into the team and they've been tight ever since, despite Gambit's secrets putting a strain on their relationship.
Chase Stein and Karolina Dean from the Runaways. They're close enough that Chase even stopped using "gay" as a pejorative after Karolina came out as a lesbian.
DC Nation takes the Troia and Nightwing dynamic and cranks it to eleven. One of the alternate universe plotlines even implied that they may have been brother and sister in several of Donna's other "short and unhappy lives."
In the same universe, Connor Hawke will say his feelings for Mia (Speedy) are like this...the truth is a bit more complex.
In Titans and the Lost Boy, Starfire apparently decides that Ryouga is similar to a Tamaranian. This leads to her growing closer to him, which eventually leads to this kind of relationship. Said relationship persists throughout the sequel, Dark Titans, and the sequel/crossover with The Road To Cydonia, ''Reflections Lost on a Dark Road''.
In the Warrior Cats fic Close, Firestar and Sandstorm's canonical romance is described as this, conveniently allowing Firestar to mate with Tigerstar. The fact that he mated and had kits with her is glossed over.
The grown up Godzilla Junior and Mothra Lea became this by time we meet them in The Bridge. They've been close friends and allies for over a decade since they first teamed up, able to read each other better than anyone. For all intensive purposes they are a platonic Battle Couple.
This is in huge contrast to how their parents got along... or lack there of.
Film - Animated
Wreck-It Ralph and Vanellope von Schweetz's friendship develops into this.
In X-Men: First Class, Charles cites this when Raven, feeling insecure about her looks, asks if he would date her... although it falls a little flat coming right after he's answered the question with "of course" in reference to her human form, before she clarifies that she means in her real form. (Charles is being a bit obvious in invoking the trope as the film establishes that she is indeed his adoptive sister.)
Lampshaded in X-Men: Days of Future Past, by the older Professor X when he mentions that Mystique was like a sister to him. It's later alluded to when a nurse wonders if the blue, scaly woman at the Paris Peace Accords has a family, and Raven replies, "Yes, she does."
On the plane ride to Paris, Charles argues that he raised Raven to be something better than a killer. Erik is quick to point out that Charles didn't raise Raven, they grew up together. This is what it takes for Charles to realize that Raven is not his to control.
In the Hallmark version of Jason and the Argonauts Atalanta prepares to confess her love to Jason and is overjoyed when he replies that he loves her too, but has a decidedly different reaction when Jason goes on to say that they are this trope (he actually says "you've always been as dear to me as a brother", ouch).
In Little Women, old childhood friends and neighbors Jo and Laurie ultimately decided to just love each other like brother and sister... which was an obvious choice, since Laurie had just married Jo's sister, Amy... while grieving and contemplating suicide after Jo turned him down.
Harry Potter: Harry assures Ron in the seventh book that he only loves Hermione like a sister. Until then it was confused for something more by a number of other characters; it was a catalyst in Harry's breakup with Cho Chang and Dumbledore makes a comment in the sixth movie about it. That's not even mentioning all their Real Life supporters; Harry/Hermione was one of the most popular ships in the fandom before Half-Blood Prince, and when J.K. Rowling shot it down in an interview with Muggle Net, it resulted in massive Internet Backdraft. (And given that it's likely no fandom has ever had a higher Crack Ship quotient, lots of people just ignoring the Word of God.)
Doesn't help that J.K. Rowling later said that Harry/Hermione could have worked out better than Ron/Hermione could even as adults.
Egwene uses this line in The Wheel of Time to make it perfectly clear to Rand that he has no attachments (He agreed, though she thought, incorrectly, that he was mostly putting up a brave face at the time). Followed by her best friend Elayne getting into a relationship with Rand that is most definitely not so. Perrin also says it in an attempt to let Min down easy, before realizing that he needn't because it's not him she's after.
Princess Andromeda does this deliberately in Mercedes Lackey's Tales of the Five Hundred Kingdoms. She and her rescuer swear blood-siblinghood to prevent the story from forcing her to fall in love with him. It turns out it wasn't as needed as it seemed, as they actually ended up swearing sisterhood.
In the Arrows of the Queen trilogy, Herald trainees Skif and Talia try for an adolescent romance but continually find circumstances conspiring against them. Eventually they give up and swear blood brotherhood instead. The inevitable shipping is sunk once and for all when she ends up lifebonding to Dirk.
In the Circle of Magic series, this is the explanation for why none of the main characters are sexually interested in each other. They call each other foster siblings.
At the beginning of Mansfield Park, Sir Thomas Bertram is reluctant to adopt his niece for fear of Kissing Cousins. His meddling sister-in-law Mrs. Norris believes raising them like brother and sister will actually be their greatest protection against that trope. She's wrong.
In Patricia Briggs's Mercy Thompson novels, Mercy's love triangle with Samuel and Adam is resolved when Samuel, a werewolf, is forced to admit that his wolf side sees Mercy this way, not as a potential mate.
A "like brothers" example in The Dream Thieves with Richard Gansey and Ronan Lynch. Local Bad BoyKavinsky makes comments throughout the book about Ronan being in love with Gansey and/or sleeping with him. When he confronts him towards the end of the book, Ronan claims that it's not like that and that he sees Gansey as a brother, criticizing Kavinsky for not being able to see more to relationships than sex.
In P. G. Wodehouse's Thank You, Jeeves, Bertie has to spend a good deal of time insisting that he and Pauline Stoker are this trope. No one buys it because they used to be engaged, even though Pauline is now in love with Bertie's friend Chuffy and Bertie is actively trying to set them up.
In Chronicles of Chaos, Amelia, who has been in love with Victor since day one, has her hopes crushed when he curtly tells Colin to "stop molesting his sister." Victor is actually tacitly warning Colin to back off.
In Ted Dekker's book Thr3e, Kevin and Sam are this. It's stated that they are best childhood friends and would never, ever jeopardize this. It becomes fairly obvious why there can be no sexual attraction when you find out that Sam is actually one of Kevin's multiple personalities.
Adventure Hunters: Lisa's feelings for Artorius are complicated to say the least. The age difference is great enough for him to be her father but they work more like siblings. Regina thinks she's genuinely in love with him.
In A Song of Ice and Fire Tommen and Margaery are married. Tommen is too young and innocent to really grasp what marriage is about, but they are technically married and he sees her more as a sister.
Arguably Jace with Isabelle, with whom he has a purely platonic relationship.
The Heroes of Olympus has at least two sets: Piper and Leo (she calls him the annoying little brother she never had) and Percy and Hazel (who gives him a "sisterly kiss" at one point).
A third set forms (or at least starts to form) between Reyna and Nico in Blood Of Olympus. Despite both being generally aloof characters, as their quest goes on Reyna ends up having to remind herself that Nico is not a little brother she needs to take care of; both share details of the darker aspects of their pasts that they've never told anyone else; and, to top it off, when Reyna hugs him at the celebration feast, he accepts it rather than pulling away, a show of affection Nico has reserved only for his sisters.
In The Hunger Games, Finnick and Johanna. While we are limited to what our first person narrator actually can observe, she considers their "years of friendship" is probably worth more in the end than their in-game alliance. They seem especially close even within the larger Breakfast Club of Hunger Games winners, just judging by the fact that Finnick can get away with hauling Johanna into the ocean and dunking her until she stops screaming. And there's no danger of things becoming romantic, as Finnick is in love with another victor, Annie.
Gale and Prim have this type of relationship. Katniss also sees her relationship with Gale as being this... until she finds out he sees things differently.
The Dragonets from Wingsof Fire. They've been raised together since birth, and think of each other as surrogate siblings. The only exception is Starflight's crush on Sunny, but she turns him down specifically because there relationship is too familial.
Buffy and Xander after Season Two. For the first two seasons it's a bit one-sided (Xander's in love with Buffy during this time).
Xander and Willow also end up like this. They start off more like Platonic Life Partners, but as the series progresses they drift apart a bit, and form close relationships with other people. They remain part of the same group of True Companions.
Cordelia and Wesley bicker and taunt each other mercilessly, but they are good friends, and are always there to defend each other in times of danger. If it wasn't for the accent difference, they could almost be brother and sister.
Cordelia and Angel were also like this for the first two seasons of Angel, which led to some viewers being displeased when their relationship changed to a romantic one from partway through the third.
In the Brooklyn Nine-Nine episode "Tactical Village", Peralta claims that Amy Santiago is like a sister to him. Boyle isn't convinced, and Peralta later admits he does have feelings for her.
However, Jake Peralta and Gina Linetti are actually this. She refers to Jake's grandmother as 'Nana'.
On NCIS, Ziva tells Ray that Tony and she are like brother and sister. He's visibly stunned (put them together for five minutes and it's kind of obvious this isn't true).
C.J. and Toby on The West Wing are this, rather blatantly, since they're in a group of True Companions that is closely modeled on a nuclear family. How they actually got to be friends (years before the show started) is frustratingly never mentioned, just like Josh and Sam, or Jed and Leo.
Mal and Zoe. They appear to have been Platonic Life Partners until Zoe married Wash, but since Zoe unflinchingly chooses Wash over Mal in War Stories, it's safe to assume that, much like Xander and Willow above, Mal and Zoe's relationship has been demoted. After Wash dies in Serenity, it's quite possible that Zoe and Mal's relationship will return to Platonic Life Partners status.
Though that might have been less Zoe choosing Wash over Mal and more her unflinchingly making a logical decision for the sake of the whole crew: she knew Mal was a lot tougher than Wash and that she could take over his duties if need be, whereas they only had one pilot.
Also, Mal and Kaylee, and their actors in real life. Leading to this exchange in the Big Damn Movie (though it's somewhat jarring, as both of them have seen each other naked):
Kurt Hummel and Mercedes Jones in Glee. Although the interest in them being a couple is completely gone by episode 3 when Kurt comes out of the Transparent Closet...they stayed the closest (platonic) friendship in the show for quite a while.
Donna and the Doctor from Doctor Who, although they were often mistaken as a couple. One of the first companions in the revival not to view the Doctor as a romantic interest, these two characters had a very close platonic friendship.
Guinevere and Merlin became this on Merlin. They probably have the healthiest relationship on the show, and both were prepared to die for the other within the first four episodes of the show.
Arthur and Morgana sort of have this, but it's debatable and their relationship evolves over the series. Considering that Morgana is Arthur's father's ward, she's essentially his foster sister. Actually, she's his half sister, so a literal application of this trope.
Luckily Mordred already exists in season one and is probably about half Arthur's age at that time, so there will be no need for either Morgana or Morgause to seduce Arthur and bear his child for the sake of the plot.
Ted and Lily from How I Met Your Mother. Notable in that, since Lily's been Marshall's girlfriend since the day he and Ted became roommates, and Ted and Marshall have been Heterosexual Life-Partners for almost as long, Ted and Lily have been basically brother- and sister-in-law since college.
On Leverage this plays out between Eliot and Parker with Eliot playing the role of a protective big brother. They even manage to have interesting conversations while on the job:
Parker: What's sexting?
Eliot: I am not having this conversation with you, Parker.
Babylon 5: Captain John Sheridan and his Number Two, Susan Ivanova, are best friends, confidantes, and comrades-in-arms, and have walked through fire together. There's also not a hint of romantic interest in each other on either side. This is played for pure hilarity when Sheridan has a midnight brainstorm and comes barging into Ivanova's quarters babbling about his brilliant plan, only for Ivanova, who's wearing some fairly skimpy nightclothes, to point out that she might want to go change first. Whereupon:
Sheridan: Oh, uh, right. Sorry, I hadn't noticed. Ivanova: Thanks, I'll just... wait. What do you mean you didn't notice?? What am I, chopped flarn? I mean, okay, granted, I don't have any interest in you, and you don't have any interest in me, but if you're going to come barging in here in the middle of the night the least you could do is say, 'Nice outfit, Ivanova!' and then go on a tear! Sheridan: [utterly flummoxed] Uh... nice outfit, Ivanova! Ivanova: [stalks away, muttering] You are such a... [indecipherable grumbling]
Jess and Schmidt from New Girl. The prospect of them hooking up has become an object of shame and disgust to them and everyone who knows them.
Friends: Joey and Monica in later seasons. Mainly because Monica gets together with Chandler, who Joey considers his "brother" and the trio form a mini-family within the True Companions. (Monica feeds Joey for years, they include a 'Joey Room' in their new house and frequently act as his older siblings).
Elaine and Jerry from Seinfeld. They tried dating, but it didn't work out. The show teases them once or twice, but they're mostly just best friends.
Warehouse 13: Despite being the two main characters, Pete and Myka repeatedly refer to each other as best friends and are fairly horrified when they believe they might have slept together (they didn't). A few moments clearly show their close-yet-platonic relationship, such as them grasping hands and merrily skipping away like a couple of schoolgirls.
Subverted by French singer Jean-Jacques Goldman in Elle a fait un bébé toute seule: "I'm like her big brother... A little incestuous when she wants to".
Tidus and Rikku from Final Fantasy X. Rikku is the first person Tidus is able to talk to in Spira and they form a really close bond over the course of their journey. Yuna and Wakka also as they grew up together on Besaid. She actually says that she and Wakka (as well as Lulu) are brother and sister.
Tidus and Lulu in a way become this as well. Lulu is initially cynical towards Tidus when he joins the group but she is always the one who tells him about Spira and she has definitely warmed to him by the time they get to Guadosalam.
Vaan and Penelo appear to be this initially in Final Fantasy XII but the end of the game implies that they might be more.
In Kingdom Hearts Sora seems to have a special bond with Ariel. They are certainly much closer than Sora is with the other Disney characters who appear in various worlds. Especially in the first game, Ariel's desire to see other worlds is paralleled with Sora's same desire from the beginning. There's good justification for a romantic pairing except Ariel is canonically paired with Eric in the second game while Sora has overt feelings for Kairi.
Falco Lombardi and Katt Monroe from Star Fox. They're not related, but they've known each other for most of their lives. Their once-ambiguously-portrayed relationship turned out to be this once it became clear in Farewell, Beloved Falco that Falco isn't into girls, but they have a sibling-like loyalty to each other.
At the beginning of Castlevania: Portrait of Ruin, Jonathan Morris states that his childhood friend Charlotte Aulin is like his dear sister, much to her displeasure since she doesn't like to be treated like a child.
Though it's never outright stated, the male and female leads in Tales of Vesperia fall into this. Despite numerous Shipper on Deck moments from other companions in various skits, they never stray from being like brother and sister.
The same case can also apply with the main hero and heroine of Tales of Graces. Despite the traditional romantic feelings harboring between the Tales of... heroes, this time, the main character's love interest isn't the main heroine (who unfortunately is hit hard by some fans), so despite the numerous Ship Tease moments, nothing romantic ever occurs between the two of them. It doesn't help that the main heroine isn't even human. In the Lineage to the Future arc, Asbel explicitly refers to Sophie like a sister (although in a skit, she refers to him like a father-figure as well - It Makes Sense in Context).
Eventually Asbel adopts Sophie into his family. Not as his sister, though, but as his daughter.
Graces also seems to be invoking this trope with one of its win quotes. Asbel refers to himself, his brother, and Cheria - who clearly harbours romantic interests for him - as "Lhant's Triplets of Terror." Cheria angrily retorts, "I am NOT your sister, Asbel!"
Miles Edgeworth and Franziska Von Karma. This one is quite blatant in that they were raised together as brother and sister, under a family name, not some cliche'd "Raised on the same orphanage" thing. Both parts acknowledge the other as a sibling figure, and Franziska has said Edgeworth was a Von Karma numerous times. This doesn't stop them from bickering, but that doesn't stop blood siblings either.
Franziska: "He's like my little brother."
Apollo and Trucy seem to fit into this trope perfectly... Until it's revealed that they, unbeknownst to them, actually are brother and sister. Half-siblings, but still...
Corpse Party has Mayu Suzumoto and Sakutarou Morishige. Sakutarou does not take it well when he finds out that the mutilated corpse he's been obsessing over is Mayu's.
In Little Busters!, if you don't get together with the main heroine Rin, she and Riki stay friends like this.
In Umineko: When They Cry, Shannon and Kanon's relationship is like this. It's eventually made clear that they're actually the same person, created as alter egos of Sayo Yasuda; Sayo even created Kanon to be a "little brother" for Shannon.
Bittersweet Candy Bowl has Mike and Lucy when they are younger, though they later develop romantic interest in one another. Paulo claims to feel this way about Daisy, and Daisy seems to feel this way about Paulo, but there are some hints that Paulo, at least, is somewhat interested in his friend. Mike also sees Daisy this way, much to her frustration.
In Something Positive, Davan and Aubrey are like this, especially since they've known each other long enough for the Westermark Effect to be in place.
Davan and Pee-Jee are an almost as good example, although they haven't known each other nearly as long and Davan had a crush on her prior to the comic's beginning. Davan even refers to her as Mei-Mei (Chinese for "little sister") at one point.
Homestuck has KarkatVantas and KanayaMaryam. Karkat is a raving, permanently angry misanthrope who hates everyone, including his morails, his matespirit and even himself, but he will never get mad at Kanaya and is a Jerk with a Heart of Gold through and through. Similarly, while Kanaya can sometimes be condescending and aloof she's mostly a Nice Girl, and rarely raises her tone to Karkat. If either needs advice or support they talk freely. Since Trolls have no formal definition for siblings, they refer to each other as "best friends".
In El Goonish Shive, when Elliot tried to assess why his recent feelings toward another girl are different from his feelings towards his girlfriend, Sarah, he suddenly realized that he sees her more like a sister than as a girlfriend, but had been oblivious to it all that time.note Ellen had realized this about herself shortly after discovering she couldn't be with Sarah but wasn't sure if Elliot felt the same way While his girlfriend didn't feel the same way about him, she ultimately wasn't too disappointed as she had her own reasons for wanting to break up with him.
Billie and Walky in Dumbing of Age. They've known each other since childhood (though Billie avoided him in high school), bicker near-constantly, and Billie seems to view Walky as her Annoying Younger Sibling. They tend to act angry or nauseated when someone suggests they could be a couple.
Even Walky's actual sister Sal acknowledges this, telling Billie she's the "...daughter [our parents] like" and smiling when Walky tells her he wants to be a good brother—to Billie—as a backhanded way of acknowledging he hasn't always been a good brother to Sal.
Da-Jeong and Min-Seong are like this in The Friendly Winter. The manga has implications of their parents pairing up so they could literally end up siblings one day.
Apollo and Calliope in Thalia's Musings, except on rare occasions when Calliope is drunk.
Also from DCAU, this is how Clark Kent seems to have come to view his ex Lana Lang, though she threatens to go back to Lex Luthor if he ever calls her that.
Lance and Illana from Sym-Bionic Titan poses as siblings in their high school. Gennedy Tartakovsky said before there was to be no romance between those two.
Terry (Batman) and Maxine in Batman Beyond have this, being really really close. Although many fans think they'd just be better off dating, neither characters actually show any interest in the other that way aside from a teasing banter here and there.
Gaspard And Lisa (from the All-CGI Cartoon based on the illustrated books) are much like brother and sister. They met at school and are almost inseparable, perhaps since their personalities click very well. Although, the nature of the dialogue-heavy drama that unfolds between them (always resolved by the end of an episode, of course) makes them look a whole lot Like an Old Married Couple despite being kids.
The alternative name for this trope, the Westermarck Effect, is after Finnish sociologist Edward Westermarck, who did very thorough research on the incest taboo and found that sibling incest, father-daughter incest and mother-son incest are extremely rare in monogamous cultures around the world, and that this effect extends also to stepchildren adopted in very early age. The same effect has also been discovered on many animals, especially birds.
This is quite common in real life amongst childhood friends. Studies show that children who know each other during their early childhood will rarely develop any romantic interest later in life. It's hypothesized that this is part of an instinctual mechanism designed to minimize the possibility of inbreeding. It's called Westermarck effect. But do Middle/High schoolers with those two friends who have known each other since preschool care? Nooooooo....
Leonardo DiCaprio and Kate Winslet, having been very close friends sinceTitanic and collaborating again in Revolutionary Road, remain this and nothing more. Shippers disagree. Kate's kids call him "Uncle Leo", and for a kissing scene in Revolutionary Road, Leonardo stated that it felt slightly weird because kissing Kate was like kissing a family member.
Harry Potter actors Rupert Grint and Emma Watson said it was weird to kiss each other for the eighth and last movie because they had grown up together since the first one and thought of each other as brother and sister. Same when Emma had to kiss Daniel Radcliffe (though the relevant scene itself was already decidedly unpleasant in context).
Arguably the luckiest of King Henry VIII's wives was Anne of Cleves. Misled by flattering portraits on both ends and a need for a political alliance, they married. As soon as it was time to consummate things, the pair found out that there was absolutely zero attraction and Henry opted for an annulment. Anne was sensible enough to sign the annulment papers with all due haste (and probably a sigh of relief); she thus kept in good standing with Henry, who liked her very much as a person even though there was no physical attraction, and became known as the "King's beloved sister." She was invited to all the events at court, and was close to both Princess Mary and Princess Elizabeth.
David Faustino denied that there was ever any romance between him and Christina Applegate on Married... with Children, instead defining their relationship as this trope.
Jennette McCurdy has specifically said that kissing Nathan Kress feels like kissing a brother, and it's part of the reason some fans see the relationship of Sam and Freddie in the 5th season of iCarly as being subject to Shipping Bed Death due to the lack romantic chemistry.
During an interview with Craig Ferguson, Jet Li stated that his fellow actress Michelle Yeoh, whom he had quite some history working with, is "Like a brother to me".
Bradley McIntosh and Rachel Stevens out of S Club 7 remarked that having to kiss in the movie Seeing Double made them react like this. In the movie Bradley is supposed to be attracted to a clone of Rachel but not the real one, and in interviews he said filming the kiss scene was like kissing his sister.
Jack White and Meg White of The White Stripes are actually ex-husband/ex-wife, but referred to themselves as siblings. Not like siblings. But as siblings. Let that sink in.
Lesbian women and homosexual men very often develop sibling-like relationships with their friends of the opposite sex.