I am a woman with aspergers and writing an autistic character has always been close to my heart. Fiera Allas, a fantasy powerhouse with immense power and sword skills has always struck me as autistic (possibly because of how much of myself I put into writing her) and I want more than just the few autistic people who may read my book to go "oh yeah she has similarities to me cool." I want to be able to give Fiera a name for something that affects her like that but this is, of course, a fantasy world. How do I make it clear to neurotypical readers without stereotyping a good character or calling her by the name "autistic."
I am not on the autistic spectrum, and I confess that it is not obvious to me to what extent and in what manner you plan to characterize your character. On the other hand, I think that your problem could be common to other types of characterizations.
To show that a character has certain features, for instance being on the autistic spectrum, there are some standard exposition tools:
Place the character in situations where that characteristic becomes obvious. Establish that in order to overcome a certain hurdle, you character would need a certain characteristic, and then show how they succeed. This is common to hero's journeys subplots.
Define the characteristic by comparison with the world standard. In particular, you first establish what the norm of your world is, or better what is already considered uncommon within the fantasy world, and then you show that your character is exceptional in that regard. For instance, if the character can run exceptionally fast, make them win a race against an opponent that is considered very fast by the standard of your world. This approach is very commonly used in the Nordic myths.
Give the character a companion that would be complementary to your character. If your character is very tall, give them a very short companion. If the character is strong and slow, the companion will be weak and nimble. (This is probably a trope in many adventure anime).
Make the character (and the reader) aware of the characteristic via internal exposition. For instance, a manuscript that explains the amazing powers possessed by certain individuals, even if they are currently unaware of them. A trope of teenager's superheroes.
Finally, and this is the frame challenge, don't use any of the above. Just be coherent in your writing. A reader that is keen and attentive to details may notice it. Most readers won't. They will notice unusual patterns, but may attribute them to what makes your character amazing. Doyle's Sherlock Holmes is a very good example of a character that is far from being neurotypical, yet, besides the occasional cocaine or opium intake, I believe that his behaviour is never attributed to any specific condition. The coherence across all the stories prompts readers to realize that he is different. They may even define him as a sociopath. Over all, they definitively root for him.
Let me suggest some examples how I imagine you could use these approaches in your context, in order:
Give a challenge that a neurotypical individual would fail. For instance, defeating a doppelganger monster (or a witch) that lures victim by enticing them with body language. Your character could have a higher resistance towards such type of traps.
Define a part of the world where people are very neurotypical. In this part of the world there is an enclave of people who are barely at the edge of the spectrum. Then your character arrives, and it becomes clear that she is far more unique than any person in this enclave.
Fiera Allas has a squire, his name is Little Brian, who is neurotypical to the excess. Most often he would just sit and stare at her in amazement, and describe to others how amazing she is, and how, even after all these years, in his eyes she is always unpredictable, and rightly so.
You character may find the manuscript describing the prophecy of Asp, the kindly Goddess, who gifted the world of individuals with unique skills. When she reads it, it becomes clear that she is one such individual.
Just be coherent in what makes her autistic. Over the arc of an long story, readers will become aware that she is not the usual neurotypical character.
I shall kick off with anecdotal evidence: when I was a kid I couldn't distinguish the sounds 'v' and 'f' as they both sounded like 'f'. I had trouble with all the voiced consonants (which are produced when the vocal cords are vibrating: v, z, j, b, d, g), but 'v'... I couldn't even hear that sound. All I heard was 'f'.
This was over 30 years ago. My primary school teacher told my mother I had trouble with some letters and that I should read a lot and do lots of copies and dictations. My mother used to tell me these tongue-twister rhymes with the sounds I had trouble with, but it still took four years until I could finally distinguish the elusive 'v'.
Nowadays, the teacher would have told my mother I had dyslexia and my spelling errors would have been excused because, alas, the poor child is dyslexic (this last part is a bit of venting against current strategies which doom a lot of dyslexic kids, sorry).
How does this apply to you? Well, just because people back then had no idea what 'dyslexia' meant, it didn't mean they couldn't tell something wasn't quite right. And I don't mean like 'they're not normal, poor thing'. My teacher knew that if I had trouble with those letters, I simply would have to work harder until I managed to get over it, something I'm very thankful for.
Back then, there were also expressions for what we today call 'hyper-activity' (it was to have the 'carpenter's bug' - I'm afraid I don't know an English equivalent) or social anxiety (you were a 'creature from the woods'). Now please keep in mind that these expressions were not negative (unless you used them in a generally negative sentence or with a negative tone, obviously). They simply named a common reality which had no scientific name.
Having had a student with Asperger, I'd say that the epithet of 'creature of the woods' would have been used on him as he didn't enjoy social activities. He'd be called headstrong and obssessive, too.
So, now, you have a choice: in your fantasy world, do people have an expression for people with the set of traits your character exhibits? Remember: the expression is not meant to be negative per se. A kid with the 'carpenter's bug' will be seen as cute by most to the exception of their parents and teachers. In young children, being a 'creature of the woods' means the child is shyer than most and the parents have to gently (otherwise it will only worsen and prolongue the problem) teach them to be more social. It may even be said with the tone of 'oh, so cute'. When you grow up, you can use those expressions matter of factly when talking about someone, though 'creature of the woods' isn't exactly used to an adults's face unless you're on friendly terms.
Another note: use an expression that is evocative of the traits, rather than something esotheric. Remember that this was a term coined by everyday people. A carpenter pestered by a bug is going to scratch and fidget because of it, and a creature (animal or person) from the woods is going to run away from people.
If you don't think the set of traits would be common enough to merit an expression, then divide those traits into groups that are more common and name those.
After all, as a kid I was a 'little creature of the woods', had a terrible case of the 'carpenter's bug' and my head was always in the moon (the term used is literally 'moony'). We call the conjunction of the last two ADHD these days, and the first we call it social anxiety.
If you really want people to associate those traits with autism, though, say so in a note at the beginning of the book. A lot of people know nothing about autism outside the 'rain man' film kind of thing. If you want people to really know, you'll have to spell it out and actually educate the readers about the condition, but do so outside the narrative.
Asperger's is not an on/off digital status. It's a gradual thing, some are more than others. You don't need to label your character, just indicate that there's something not typical going on, but, hey, we all know people like that.
Anyway, people I know who are - um, whatever - typically say things like "I don't do people" or "I don't do touching", or "sorry, I'm a bit detail-obsessed." They will avoid parties or leave suddenly when it all gets too much. They will sit for days over a problem, totally focussed. If you tell them a joke you have to warn them that a joke is coming. These are things your character can say and do.
You'd need to highlight the challenges you think most typify the autistic experience. I'm an aspy my main personal challenge is reading body language, I have other issues but that's the one that has traditionally caused the biggest hassles. That's a common trait for those on the spectrum but by no means universal you need to look at the traits you find most telling and convey those. First person POV and internal voice are, to my way of thinking at least, going to be essential to this task. Most of the information you need to convey is in the form of the thoughts and feelings of the character being portrayed. Hopefully that helps you, don't be surprised if you can't get the effect you're looking for, a lot of people just won't get it unless you slap them in the face with the terminology, and maybe even then.
You know, I never thought I'd see another writer who attempted this kind of thing as well. I've actually written a fair few autistic characters in fantasy, and naturally, yes, there's no word for 'autism', or any kind of formal diagnosis (psychologists tend to be a rarity in fantasy 'verses).
First thing you need to remember is that autism is a spectrum. There are various manifestations of it, and sticking to the simple 'bad at socialisation but a savant' trope is personally kind of tired, as it treats what can be a truly debilitating mental state as some kind of psychological min-maxing strategy (I only have 1 charisma, but 20 intelligence!).
I've written three autistic characters, incidentally all women. It should be noted they're not just defined by their autism, however; the personalities they exhibit are complicated maelstroms of how their neuroses interact with their circumstances and people in their life. As such, you get three people with notable harmony in some traits, but extreme dissimilarity in others. Here are my three examples:
Magaral Kazaros: A girl of twelve who is simultaneously shielded from and drenched in horror. She's a bastard child to a sadistic mother who is in essence a humanoid black widow; if she wants a child, she has no need for relationships or marriages, she just has her guards kidnap a man, rapes him, then kills him. Magaral is the second child resulting from this process (that her mother has allowed to live, at least; it's notable that she has no extant sons). Her older half-sister, Gelax, idolises her mother and thus relentlessly bullies her 'soft-headed' sister, even molesting her because she believes Mag to be incapable of fighting back.
Fortunately, she does have some kind of support network; her mother's younger sister despises the family situation and while Gelax eagerly participates in her mother's depravity, their aunt shields Magaral from doing the same. She quietly encourages and preserves Mag's kindness, and with the help of her cousins, even helps her develop a constructive hobby (harp-playing, which she takes to like a duck to water). Due to being raised in a polytheistic household but receiving kindness and harps from her highly religious monotheistic cousin-once-removed, Magaral's view on religion is... interesting. She legitimately enjoys hymns and comes off as pious, yet she has no concept of religions being mutually exclusive; she believes in both polytheism and monotheism at once. Due to all these issues together, Magaral comes off as... highly stunted, very innocent in some areas while disturbingly blazé about certain horrors (like her mother's 'games'). She's clearly passionate about things she obsesses over (like harps and hymns), but for the most part has an odd... glazedness about her, as if she's taught herself not to fully process the horrors around her. As such, the only people who fully understand the off-kilter sentences she speaks are her aunt and cousins-once-removed.
Salekh Moonchild: A young woman in her late teens who has lived a mostly spoilt life, tinged with touches of reality. She has a hard time understanding the abstract, and often takes idioms as concrete, but thankfully, she's raised by a scholar who is more than happy to help her with that. Her spaciness and general excitement to be in company makes her easy to take advantage of, and one of her 'friends' spends most of her time calling her 'fat' and 'stupid' (she's merely curvaceous for her race, and has a different kind of smarts). She's profoundly lonely, not least because her mother died when she was just about old enough to remember her, and her father selfishly committed suicide rather than stay to raise his daughter.
As such, her parents' retainer and scholar, Aldridge, is the only parental influence she has. He's a patient and kind teacher, and showers her with praise when she overcomes her mental blocks, leading to her being happy in herself, albeit a little ditzy and concrete. She's somewhat aware of her shortcomings and tries her hardest to read between the lines, but it takes active effort from her. However, thanks to Aldridge and a close female friend (who she's oblivious to the fact she has a crush on her), she has a healthy, measured self-esteem.
Kagura Selenia: A do-nothing overlady who leaves absolutely everything to her inferiors, the outside world is entirely optional to Kag Selenia. Her autism is likely comorbid with malignant narcissism, which is not too surprising given her noble line is notoriously inbred. Despite some of her historical relatives having cleft palates and six-toed feet, Kag herself is incredibly beautiful; possibly the most beautiful woman in the kingdom, especially according to her. She was not raised or groomed to rule; instead, her father repeatedly made love to his sister-wife, praying for a son that she could marry and act as a consort to, but instead, all he got were three daughters, the last of which killed his sister-wife in childbirth. From this point on, something in her father snapped, and he spent his days lavishing affection onto Kag while neglecting his younger two daughters (which incidentally turned out much better than her). As a hobbyist artist, he indulged her budding narcissism by painting her over and over again, shielded her from the outside world and left her in an echo chamber where the smallest achievement or positive trait was lauded as exceptional.
As such, Kag now believes she's some form of ubermensch, entitled to worship for her mere existence. Now that her father is dead and she's left ruling her lands to a harsh, competent scholarly figure, she's starting to be faced with little snippets of reality, or cases of her subordinates calling her out for her inaction or utterly self-serving, capricious policies, and she doesn't like it at all. Because her father convinced her that dead people float as spirits to the moon, she's convinced her dad is up there, having sex with his sister-wife, ready to send her a brother from the moon so she can finally get married. To this end, she's prevented her sisters from marrying and potentially making an heir before her, and pushed her scholar-regent to perform unethical experiments to find an elixir of youth that will keep her young and fertile for her brother who is totally going to arrive soon.
As you can see, these three all have some shared issues (obsession is shared between Mag and Kag, ditziness between all three, being easy victims of bullying between Mag and Sal), but they're undoubtedly different in result. One is a creepy semi-savant with an innocent core, one is simply a self-aware ditz trying to account for this, and one is an outright delusional idiot. You can take autism a number of ways, but the key is to remember that not all autists are created equal.
Does your character really need to be labeled? Write her out of your own experience, and people who share that experience will recognize pieces of themselves in her --as will people who don't share that experience.
I have a child with a diagnosis on the spectrum (and my wife always tells me I should have been diagnosed as a child!) and while the label is useful for gaining extra support, and to help people conceptualize his way of approaching the world, that's all that it's good for. It doesn't really say anything deep about who he really is as a person. It's just the way our particular culture, at this particular point in history, helps itself understand a portion (a sub-spectrum, if you will) of the incredible range of human diversity.
I don't see a fantasy world as necessarily having that same need. But it's your world, and you can write it as you want. So think about how that world might conceptualize ASD and what usefulness a label might have for it. Maybe that range of traits is always associated with magical gifts, or weaponry skills, so they call people with those traits the "gifted ones" or something along those lines. Or, if you don't want it to be so positive, maybe it's a set of traits that people recognize and fear, and therefore try to hide. The possibilities are wide open.
Depending on your setting and if you want your character to be more or less different from neurotypicals. You can create a new word for Aspbergers that fit your world.
"When she grew up it become clear that she was a Nothlak. Many of the mages and scholars in the capital were Nothlaks, as the mages require years of studying and undisturbed concentration. Often wearing robes, and gloves as they are sensitive to the touch."
Create an in world name for the "diagnosis", and put a fantasy spin on it. I mean a fantasy dwarf trope is that they are immune to magic. It does not need to be in a negative way, like Nothlaks are maybe a bit odd to the common man, but may have some strengths that can be considered divine or special that "makes up for it".
Since it sounds like you already know how to write a well-rounded autistic character without relying too much on labels and simply want to make sure readers who may not know much about autism learn something from what they read, I would suggest the simplest solution: use the label autistic in the text itself once or twice.
A fantasy series I read regularly (Pathfinder) recently started using the words asexual, agender, and intersex to describe characters where appropriate. I know that these words for sexuality, gender, and sex were not in common usage during the Middle Ages when knights were running around. It surprised me the first time a writer used the word asexual in the series, but it didn't break anything. Yes, the authors also showed that an asexual character wasn't interested in sex in other ways, but since asexual people are so often told their orientation isn't a real thing and "they'll find the right person eventually", it was refreshing to have the label explicitly stated. I know that character's identity won't be taken away in the future.
I probably wouldn't use the term Asperger's in a fantasy setting as the name is a reference to an actual person. If you wanted something similar to that term, or if the word autistic sounds anachronistic to you, you could use a name of a fictional character you create or do some research into the time period that you are basing your setting off of. Are there any historical figures from that period that you think would be considered autistic today? How did people refer to them during that time period?
Alternatively or in addition, if you put your own place on the spectrum in the author bio, readers will be more likely to see the connection. You could even mention in the author bio exactly what you did here, that you see that aspect of yourself in Fiera.
Give your character a "normal" companion
Alas, there is no "name" for autism in a fantasy world. Of course you can invent a name for it, and even make autistic people known for various talents. Hey, you can even make them eligible for a special Hogwarts-like school.
If you want to follow "show, don't tell" practice, you need to reveal your character's condition in interaction with other characters. Placing another character who would appear to the reader as "normal" would immediately highlight what is different about the main character.
Since nobody else has suggested it: take the easy way out, and have them encounter a psychologist who was magically transported from Earth somehow who can formally diagnose them - the other elements typical of isekai fantasy being optional. That way, you can explicitly label them as autistic within the world, without having to explicitly state that using the authorial voice.