Yotsuba's an alien, possibly related to NarueThis explains the green hair and lack of familiarity with common items. She spent a lot of time star-watching on the starship she came here on, and assumes (in the Yotsuba & Stars chapter) that the brightest star in the sky is Earth because that's what it used to be when she was going here.
Yotsuba is a war orphan from a third world country like Bosnia or Croatia.This is why she is "invincible": she has seen hell, so everything else is heaven. It also explains her constant fear of the "enemy".
Other origin theories
Yotsuba is a blond American with chlorine-damaged hair.Koiwai mentions that he met her "abroad", which in manga usually means America. At Fuuka's school, someone notices that Yotsuba is a foreigner and says "hello" to her in English. Her green hair? Chlorine. She's been shown to be an excellent swimmer, so she must have spent a lot of time in chlorinated pools.
Yotsuba is a leprechaun.See above. She acts as if she is invincible because she is a magical creature and therefore is invincible. She changes the world around her in subtle ways, probably subconsciously, making it a benign and friendly place to live. She doesn't show any other characteristics of being a leprechaun because she has been raised by a human almost her whole life.
Yotsuba is a changeling.The spell to disguise her as a human baby wasn't cast properly, leaving her with green hair. Her birth parents were really freaked out about her because of this, and essentially abandoned her. (Which is entirely compatible with several other WMGs.) Now, to support this, she's obsessed with festivals, is never seriously injured, even when it appears she will be, and claims to have moved from "the left", a direction associated with the Fae.
Yotsuba is Alpha.v.0.1. Mr. Koiwai is the Owner. After all, both have green hair. I just saw "Irish people don't have green hair" and saw YKK under it for the previous WMG and...
Yotsuba is somehow related to the Yotsuba GroupI don't know how, just desperate to make the connection. Maybe Alpha is Misa and Rukia is...Takada.
"Kiyohiko Azuma" is a collective pen name for AnonymousShe, and the entire series, was created just so that 4chan can have a mascot.
Yotsuba is a young Haruhi SuzumiyaShe only gets weirder as she grows older... And she loses the Implausible Hair Colour.
The entire series is an extended exploration and Deconstruction of a possible route a girl might grow up to be the main character in a Shounen animeYotsuba lives with her (adoptive) father and has picked up many of his and his (male) friends' mannerisms. The reader is introduced at the turning point in her life, when she begins to get the chance to learn, from the Ayase girls, the cues and behavioral patterns that differentiate between male and female. Although the series hasn't progressed far with these developments yet, it's obvious that, in ten years, she'll be a perfect Bokukko or Tsundere for some Shounen anime series, right down to the green hair. Her lack of past is deliberate Lampshading of this true motive behind the series.
Yotsuba is literally a Messianiac Archetype.Yotsuba never had parents—she was granted to Mr. Koiwai by the gods to raise and teach the ways of Earth so that she'll be knowledgeable when the time comes for her to prevent the Apocalypse/lead men to Nirvana/whatever. The reason he doesn't talk about her past is to maintain The Masquerade and keep everyone believing that she's normal. Also, if people knew she was The Chosen One, they would then try to influence her and lead her down the path of evil. This also explains why she seems to know nothing about anything—not only is she young, but she is also, literally, new to planet Earth. And why she loves everything and everyone—it's in the job description. It would explain the whole "green hair" thing.
Yotsuba&! is a prequel to Azumanga DaiohWhen Yotsuba grew older, she cut off two of her pigtails, dyed her hair orange, and changed her name to Chiyo. Just compare pictures of the two.
In relation to the above guess, and with some further explanation, Torako is Sakaki's older sister.The Sakaki parents, being well off, sent Torako to a boarding school after Sakaki was born, in preparation for her (hopefully) getting through college on her own, as did Sakaki later. Sakaki looked up to Torako's idealized image and thus became enamored with house-cats (tigers' children, to a small child) and cats in general, but after Torako became less unfamiliar thanks to her interactions with Yotsuba and subsequent interactions with Sakaki, Sakaki lost the Un Favourite complex but kept the cat fixation. Yotsuba losing the nickname, dyeing her hair, and using the parental-designation-as-nickname from Mrs. Ayase for the live-in carekeepers (or extended relatives) while Koiwai is busy at the UN or working on some huge corporate translation project (perhaps heading the translation project for Azumanga Da Yo, from which Fuuka's Chiyo-chichi merchandise and Chiyo's stuffed animal originated) all add up to Sakaki making no connection between Yotsuba and Chiyo, and Sakaki never talking about her family equals Chiyo making no connection between her and Torako, as well as neither Torako nor Yotsuba and Koiwai ever giving their family names. However, Chiyo-chichi may be a result of a subconscious connection between the details Torako happened to mention about Koiwai when telling Sasaki about her friend's family and weird neighbors.
The police lady Yotsuba meets on her flower rampage is Tomo.She did say that she wanted to be in the ICPO. After all, she looks the part when she dressed in the uniform for the costume race. Maybe she mellowed out some in order to get her dream job?
Yotsuba is Oasis.She caused whatever disaster made her adoptable. In a couple of years, she'll be taken by The pyrokinetics facility, and they won't bother dying her hair green (or will start dying it red). Sluggy Freelance actually happens in the future, after a common way to keep people looking in their early 20s for years is discovered (as well as Owl Delivery).
Yotsuba will eventually make her own Manga about DanboIt will happen one of 2 ways
Yotsuba becomes Nel from Bleach
Yotsuba is a nature spirit, sent as an emissary to humankind
'Yotsuba' isn't her real nameOkay, picture it; in a third-world country, a 2-3 year old girl loses her parents. She's too young to have formed any concrete memories of her parents, or even what her parents called her. With no friends or relatives to take her in, she wanders the streets, surviving on scraps for months. Then along comes a young, idealistic Japanese man, eager to put his skills as a translator to work in a place where he can actually make a difference. Be it by chance or fate, the two cross paths. The man offers the girl a sweet as a token kindness, which the girl gladly devours. Upon closer inspection, the man notices the girl's incredibly disheveled appearance. Without knowing this poor girl's circumstances, and being at a complete loss for anything else to do for her, he figures the least he can do is try to wash away some of the grime off her face and hair. The girl goes along with it, if only in the hope that the strange man will give her more food. After washing her hair the man tries to do something to fix the tangled mess it has become, but his inexpert hands somehow manage to tie off the girl's hair in four separate place. The result is four oddly-shaped lumps of hair sticking out from the girl's head. Chuckling slightly at his handiwork, the man remarks that the girl looks like a 'Yotsuba', a four leaf clover. The girl was only partially aware of the concept of 'names'; in her experience they were something that only applied to other people. But the the man's kind smile and the warm feeling behind his words make something go 'click' inside the girl's tiny head. Thoughts of warmth, and happiness, and belonging flooded every corner of her mind. She thought of the man's gentle touch as he washed her face, and of the sweetness of the candy. But among all these feelings, one idea dwarfed them all. An idea of such colossal importance it might as well be the meaning of life. Which, for this little girl, is exactly what this idea, this question, this hope represented. 'I'm a ... Yotsuba?'
Yotsuba is an OniHer horn(s) have been damaged and haven't grown back in, so she can't fly. Yet.
Yotsuba is literally a four-leaf cloverShe's suffering from a massive spiral energy overdose, due to Simon's final fight accidentally brushing a nearby galaxy. This is so obvious I see no need to further expound on it; more importantly, how long will it be before she starts warping reality through sheer cuteness?
Yotsuba is the Anthropomorphic Personification of cute.This is Self Explanatory.
Even after she grows up, Yotsuba will still be defended by 4Chan.I'm not sure how. Maybe she'll just look that way forever, maybe she'll grow up to be a Memetic Sex Goddess so hot that people won't be able to comprehend that she could be hotter without clothes. Because...well, 4Chan clearly has some standards.
Related to the above guess, Yotsuba will grow up to be a Hot Gym Teacher.That way she keeps the shorts and t-shirt ensemble and the boundless energy doesn't stagnate sitting behind a desk for eight or more hours a day. She also gets to keep the Malapropers, since Azuma-san has shown that even the wisest, educated gym teachers can fail at linguistics. Her character thus takes a completely different branch from the "hotter without clothes" type, to the "would be a completely different character (who just happens to have green hair) without clothes" type.
The series will change focus when Yotsuba goes to school.As of this writing, its mid-October in the Yotsuba&! manga, and Yotsuba will start school the following April. From that day onward, the manga will be less about Yotsuba discovering something new at home with the Ayases and more about her interaction with her teachers and fellow students. If Tomo or Osaka are any indication, putting an imaginative and energetic girl into a school setting could produce interesting results.
Yanda is a member of 4Chan.What more needs to be said?
Miura, Shimau, and Torako are all the same person from different periods of time.Look at the similarities.
Asagi and Torako are lesbians.It would simply make sense, in this troper's opinion. Additionally, Yotsuba & Adolescence would seem to imply that Torako finds Fuuka attractive as well.
Yotsuba and Grave of the FirefliesThe universe caused Yotsuba&! to be created to restore the universal balance of happiness that was thrown off by Grave of the Fireflies.
Yotsuba can turn peoples souls into tang...... just by hugging them. Because she is anthropomorphic personifcation of cute, as explained in an earlier WMG, and thus is so adorably heartwarming. That means that anyone who hugs Yotsuba will find that there truly is love, happiness and hope left in the world, and then there hearts will melt and become tang. the troper who made this WMG does not watch Neon Genesis Evangelion and has only heard of the "turning into tang meme", so if it was used improperly, do not start spazzing.
Fuuka reads Azumanga DaiohIn the 5th book, Ch. 29, Yotsuba is helping collect the sheets for the Ayase's (you might have to find a physical copy for this because you can't see the back covers online for whatever reason... trust me on this one). Look carefully at what Fuuka is reading when Yotsuba gets to her room. On the front cover of the one she's holding is a humongous circular speech bubble ◊ . On the back covers of the books lying on the bed are what could pass as the outlines of Osaka (particularly these: ◊ ◊ ). This could explain why she has Chiyo-chichi stuff.
Kowai is Rock in the future. Yotsuba is an orphan from Roanapur, and Jumbo is an amnesiac Ginji.One day, Rock finds orphan Yotsuba on the streets of Roanapur. He takes her in but is eventually forced to choose between giving her away to a better home or leaving the city for some place significantly safer, which he chooses to do. Yotsuba states that she's from "way left," which Thailand is in comparison to Japan. She thought Roanapur was an island as a very small child because of the water and all the boats. Rock, adopting a new identity as innocent translator Koiwai, takes her back to Japan, where he runs into Ginji, who survived his injuries but lost all of his memories except for Revy calling him "Jumbo." Out of sympathy, Rock/Koiwai creates a fictional past for them to share and finds a childless florist to pose as Jumbo's father so he doesn't go poking about trying to find his real past. A sign of his complete transformation into Jumbo is his nervousness around Asagi. Like Yukio, she's in college, and he's subconsciously afraid that if he gets close to her, she'll die on him like Yukio did. Because of this, Rock/Koiwai tries to be as vague as possible about his past so Ginji/Jumbo doesn't accidentally overhear something that will trigger his negative memories.
Torako's family name.Many fans say it's Takino.
Koiwai is named for Koiwai Farm.http://www.koiwai.co.jp/english/ Could this be the place in the country Yotsuba supposedly came from? (meta) Is Koiwai's name a reference? (in-universe) Is it an alias to sound more country when he's really an alien spy?
Yotsuba is the grass dog.Not literally, but just like the grass dog in The Circus of Doctor Lao she is a hybrid of animal and vegetable, the final stage of evolution; lacking reproductive abilities because she does not need them. She's the most perfect form of life, and instinctively knowing this makes her endlessly happy. Just like the grass dog. (Well, she does have green hair and at one point stands in a flower fridge.)
Sakaki will work at Jumbo's florist during uni.While studying to be a veterinarian. Considering both Asagi and Sakaki are super sexy, he might try to pull some moves on her, though.
Yotsuba and Chiyo are biologically-engineered psychic supersoldier prototypes.They were the products of an experiment in human genetic engineering, funded by the Mihama Corporation and carried out in a former Soviet republic, on an island in the Caspian Sea. Due to the language barrier between locals and Japanese, the language used at the lab was English, explaining Yotsuba's Gratuitous English and Chiyo's proficiency. Chiyo was an early-generation prototype; after the corporation found that her only notable trait at age 1 was a prodigy-level intellect, they decided to ship her to Japan and observe her as she grew up. What they don't know is that she has the ability to unconsciously implant ideas in susceptible people (read: Osaka and Sakaki), which explains why both of them identify the strange cat-thing as Chiyo's father; she never had one, and the cat-thing is the psychic projection of her subconscious image of a father-figure. Yotsuba was a much more sophisticated prototype, gullible and easily-controlled but with far more destructive powers; the green hair is a side effect of the process, and may indicate the presence of chloroplasts (allowing her to photosynthesize and hence to operate without food in an extended combat situation - ever wonder why she always seems so full of energy?). Mr. Koiwai, being an investigative journalist, found out about the Mihama Corporation's lab and attempted to infiltrate it (possibly getting a job as a translator) to find out what was really going on; when an accident caused Yotsuba's powers to go out of control and destroy a large section of the lab, he rescued her and escaped. Koiwai was later approached by agents of the corporation who offered him a bargain: if he didn't publish anything on them, they would help Yotsuba to live a normal life as his daughter. For her sake, he accepted - this is why the manga emphasizes the wonder of a "normal life" so strongly, and also why, despite doing a surprisingly small amount of actual writing, he seems to be able to support both of them fairly well. Of course, the Mihama Corporation is still watching them very closely - what are the odds that someone close to them is in the corporation's employ?
The guy in 'Yotsuba & Cameras' really was the Nio.That guy really was the Nio. He didn't deny it when Yotsuba asked him or even act confused at what she meant. The Nio was checking up on Yotsuba to make sure she was still being truthful. He scolded her for taking pictures without permission to which she apologized and didn't try to lie her way out of it. Even so, the Nio still roared at her and gave her a good scare. Maybe he was a little disappointed that she didn't lie after all...
Yotsuba is Chiyo's biological daughter and Jumbo is Chiyo-chichi in disguiseChiyo settled down and got married in America after she completed her studies, and gave birth to Yotsuba. Shortly afterward, Chiyo (along with her husband) was killed just as Osaka predicted. Young Yotsuba, who escaped death by hiding nearby, witnessed her parent's murder and would eventually repress the memory. Since Chiyo was a somewhat wealthy Japanese citizen, and her family may have held some influence in Japan, a Japanese official of some sort was involved in the investigation, and Koiwai worked as that official's translator, which is how he met and adopted Yotsuba. Chiyo-chichi, distraught that he was unable to save his daughter, disguises himself in a giant human suit and passes himself off as Jumbo and dedicates his life to watching over his surviving granddaughter.
Yotsuba has a fungal hair infection from her life in the country to the left of Japan.It's effectively non-transmissible, all it harms is the terminal length of Yotsuba's hair, and it would be extremely expensive and kind of traumatizing to shave Yotsuba's head and chemically treat her scalp and follicles, so Koiwai and his pediatrician and/or dermatologist decided that it was a reasonable option to leave it untreated until further development.
Yotsuba is completely bald from nuclear testing in her home country.That's no hair, it's a symbiote!
Yotsuba has been adopted by Dr. Tenma.Think about it...when we last left him, Tenma was working with Medecins Sans Frontieres, where he met a war orphan who would be adopted as Yotsuba. Since he came back to Japan, he changed his name legally to Koiwai to avoid attention re: the events in Monster. His 'translation' work could easily be related to telemedicine or medical transcription, and Jumbo and Yanda were fellow volunteers with MSF.
Yanda is secretly a nice guyWhen Duralumin loses her voice after a dog attack and needs surgery, Yotsuba becomes near-catatonic with depression. Who gets her to cheer up but Yanda, with a series of soap bubble blowers of escalating coolness? Of course he keeps the coolest one to himself to rile her, but not when she bangs her head and won't stop crying. One begins to suspect he goes out of his way to antagonize her for her sake: Being an outlet for her frustrations, add variety and spice to her day, make her feel like the good guy, that sort of thing. Essentially Yanda is Kelso.
Yotsuba is actually six years old, not five.Yotsuba knew how old she was (six), but Koiwai lied about her age to give her an extra year of education and cultural development before she starts school.
Why this bulls-eye balloon scares Yotsuba like thatProbably cause she witnessed a massacre against her ... home village or something, where people were made to wear bullseye-masks before being shot. Yotsuba's psyche did suppress the details, but generated a great phobia against anything bullseye-mask-like.
Koiwai was classmates with the Azumanga Daioh crowdKoiwai was one of the non-descript male characters in Miss Yukari's class. Years later, after graduating and traveling the world, he met an orphan who reminded him of his old classmate Chiyo. He obviously had fond memories of his old super-genius classmate and the resemblance between Chiyo and Yotusba drew him to her and made him want to take care of her.
Yotsuba is Suu and Kazukiho's daughter.And she really is a 'four-leafed clover'.
Yotsuba is Weboshi from NichijouShe got older and went to the school full of weridos, since she is one herself. Here a screencap of her.
Fuuka is DoomguyThe reason why she exclaims Jumbo is Huge!. Presumably, the manga skipped the part where she declares "You must have huge guts!".