I'm afraid I don't know that fandom, so I can't help you directly, but I commend your dedication. It took me about six years to swallow my perfectionistic fears and post my work, and it was only thanks to some very good 'net friends that I've since built up my courage and am trusting myself - and yet I still find myself clinging to the feedback from my betas.
(By the way, you might want to start by changing "research" to "researching" in your first line.)
As for your actual request, our community isn't exactly a free beta-reading service; we do favours for each other, yes, but generally our lives are busy and we stick to writing missions because that's what we love best. So it's really up for grabs as to whether anyone around here has both the time and background knowledge to give you a hand. If no one can help, might I suggest some other methods?
Firstly, there are various litmus tests around which you can take to help judge it yourself, some of which cater to original characters, which can therefore be adapted reasonably well to any fandom for which no specific test has been devised. A few seconds of googling found me this, which looks pretty good, at first glance. If it doesn't work, just do some searching on "Mary Sue litmus test" until you find something that does work.
You could also browse around fanfic archives specific to Assassin's Creed and ask some active writers whose work you think well of if they would mind reading over yours and giving a brief opinion - most people, I've found, are more than willing to give up a little time (especially if you compliment their work first, but I didn't say that).
Or, if there's absolutely no other option, you could simply dive in and see what happens. I suggest this, actually; the worst thing you could get is a bad review, but if you put a polite, succinct author's note at the start saying you would really appreciate honest feedback because you are so worries that your OC might have some Stu-traits, you will almost certainly get feedback. It won't all be good, but some will, and that's what you're after, right?
Also, if you're anything like I was seven years ago, then I suspeect you may feel like you have to present something perfect right away or else your penname will get a not-so-great 'reputation'. If so, let me say that it's an empty worry. For one thing, this is the Internet - you can always change your name and start over. But even besides that, you've got to feel comfortable with your work because you like it. If you stake your feelings on other people's opinions, you're forever going to be a slave to them. Trust me. That's not how a writer should work. To be happy with it yourself, simply be honest. Ask yourself the hard questions. Would you be willing to completely scrap your favourite scene, plotline, or character if it was bringing the story down? Are you putting too much of yourself or your wishes into the story? These are the kinds of questions the litmus tests will make you think about, even if they don't apply directly. They're probably your best bet.
... I've rambled on. Oops. Sorry; tends to happen. I do have one last question: Are you joining our PPC community, or are you just dropping in to ask for help this once? Either is fine, but it would be good to know.
That said, nice to meet you :)
- Sedri