One big thing that I noticed about her backstory is how much it seems like you're trying to avert Mary Sue tropes. And that's okay. There's nothing wrong about that, so long as you do it in moderation; otherwise, you'll veer too far in the other direction over to Anti-Sue, which is just as horrid to read about as her perfect twin sister because she's too abrasive to relate to. Make sure that she has a few redeeming qualities to balance her out.
Also, when you really, really get down to it, a Mary Sue is a character given special privileges by the author, part Spotlight-Stealing Squad and part Wesley. When you actually do start to write, reread to make sure that she does not fit those parts and that other characters have lives and motivations beyond doing stuff for your heroine. Make sure that these other characters share a role in the plot alongside her, too.
On the other hand, you've got some good things going here. In terms of the "sex worker" stuff, I would say that what you have is okay given that it's sort of a Fantasy Counterpart Culture. Almost every last civilization on Earth has had prostitutes of some sort, and they are usually looked down upon as a source of venereal diseases. Is that a problem in the Inuyasha-verse? I don't think they'd ever say. However, it does make sense, which is definitely a good sign.
"Jack, you have debauched my sloth."
I haven't run this particular Original Character by anyone (in full anyway) for a disturbing while (which, for this author, is a span of about eight months or so). I'm rather confident that I've killed off all of her previous Mary Sue traits, but I'm well aware that I need a third party to determine that. I'd really appreciate your thoughts on her, and any feedback or criticism you have. Before I start, I should mention that this character is in an Inuyasha fanfiction that I'm writing.
I'm not going to do this list-style because it takes way too damn long. You get paragraphs instead. I'm sure we can all deal with this.
The character's name is Akiko - she has no family name, despite having a family. She is Japanese; she's a half-human, half dog-demon hybrid. Like Inuyasha (she's his older half-sister - yes, I see the cliche here - don't worry), she has the gold eyes, white hair, dog ears, fangs, and talons. That, however, is where the appearance similarities end. Her clothes, unlike his, are ragged, and do not fit properly. She dresses like a peasant, and would be otherwise indistinguishable from any other peasant woman in the series if based on clothes alone. She has the physical, mental, and emotional age of a 17 year-old, whilst being around 220 in human years. That being said, she acts and thinks like the typical self-absorbed, jerkish teenager. She makes stupid decisions, and can't really forsee consequences very well. (And I can see that I'm getting off-topic. Bad author! Bad! Let's move on.)
Akiko has the same father as both Inuyasha and Sesshomaru. Unlike the other two, Akiko is completely illiterate, and recieved nothing by way of an education. Akiko, as a way to cope with the extreme poverty in which she lives, builds an alcohol depnendence that will come back to haunt her later when it becomes her only coping mechanism for an abusive marriage - which she plays a heavy hand in as well. Akiko is sarcastic, quite rude, and generally just plain bitchy. She's extremely greedy, motivated by a lust for material comfort and wealth (and the emotional comfort and sense of stability she mistakenly thinks this will bring.) She does not watch her tongue, and this often gets her into trouble for mouthing off where she really should not (as an example, she does this to the Council of Elders of the Demon Wolf Tribes when she requests a meeting with them. They...do not take it well, nor should they.) She does not like helping others if there is no benefit to her, and derides her little brother for his "little Jewel quest." She places her survival first above that of others, with the exception of her child. But that'll come later. Akiko has no special magic powers or skills of which to speak - she, like Inuyasha, possesses very basic demon powers, and nothing more. She prefers to use her fangs and claws in fights as opposed to her fists or a weapon. She eventually takes up a short knife as a means of self-defense later.
After a particularly bad choice to semi-permanently have herself transformed into a human for safety reasons, she, in desperation to end her continuing poverty, becomes a sex worker.
A note to the history buffs: I'm aware that the "becomes a sex worker" bit is sketchy. Unfortunately, I can't get a clear fucking (excuse my language) answer on prostitution in the pre-Tokugawa era. All I have to go on are vague references and no leads when I do research. All I can find information on are the high-class prostitutes, the call-girls, and that's not what I want. That's not Akiko's job - she works as a simple, largely unsophisticated sex worker. I own the otherwise wonderful resource of William E. Deal's Handbook to Life in Medieval and Early Modern Japan, but if any Tropers have any other resources they'd like to recommend, I'm more than open to hearing them! [smile]
She does eventually learn to change her speech from rough and inarticulate to a measured, taught cadence when required (I would like to emphasize the "taught" there - this is something she had to work quite a bit on to master). With tutelage, and much patience, she also eventually learns to play the shamisen for her customers' entertainment. After a time, Akiko is visited by a very rich noblewoman, who, after a very awkward first "encounter", discovers that she's developed feelings for the girl. Akiko later comes to realize that she's developed feelings for the client, and the two build a genuinely affectionate, loving relationship. Basically, she's Bi The Way.
Much later in the story, Akiko is married (to Kouga - yes, another cliche, I know), and has a daughter. Their marriage is...rocky, to say the least. Your time is better served by reading the summary of their relationship here: http://answers.yahoo.com/question/index;_ylt=AtKKBuCZOJA2QID.6PgdJ.7ty6IX;_ylv=3?qid=20110110202051AAWXAXj
Akiko and daughter go out on a small day trip and are caught in an ambush. Daughter dies, and Akiko loses her left eye and right (dominant) arm at the shoulder. She has a very hard time coping with all of this, and isn't quite sure how to deal with the loss of her arm and the limiting of functioning that her injuries and new disabilities present. Much later, she loses her other eye to an arrow that even she acknowledges shouldn't have hit. Unlike Sesshomaru or Inuyasha, neither limbs nor eyes magically grow back. After her Healing Factor essentially exhausts itself (after the loss of her first eye and the limb), the wounds she recieves (including those) leave disfiguring scars that do detract quite a bit from any "beauty" she had - she also gained about 25 pounds of weight during her pregnancy, and was never able to take all of it off afterwards.
By the end, she is suffering from severe depression.
That's basically her in a nutshell. It's a bit rushed, I'll admit, and I did skip over parts of my plot for the sake of expediency.
But, aside from my ramblings, what did you think of her? How does she sound to you? Is there anything here that you think needs clarification? Better yet, criticize her! Please! I know this last one might sound a bit like begging, but what did I do right, or, rather, what did you like?
Thanks so much!