Hey there @
Dark Pasts
I have a couple of things I'd like to mention. First of all, your OC's eyes seem... unnecessarily overdone. It really just comes off as an attempt to make them stand out without making them really unique (this effect is compounded by the fact that she has Rainbow Dash's manestyle). In regard to the family's move when she was 9... why would being Canterlot Guards necessitate moving to Saddle Arabia? And, for that matgter, how would one come to be excessively wealthy (at least, that's how the family seems to be portrayed) by being simple guardsponies? What does it mean that her uncle was "Dark"? In what way was he intelligent? Did this have any effect on her, what was it?
In the cutie mark section, you state that her cutie mark is a wedding dress that she saw and liked. When creating a cutie mark, keep in mind that it should reflect a bit of the pony's natural talent. Think of the Mane 6 and how they received their cutie marks. They got them by doing something at which they would come to excel. Based on the story given, all I could think of for a talent for Valiance is that she's really good at thinking about clothes. I suppose that's a claim that you could make but it seems rather unsatisfying in terms of creating a character. As a side note, instead of trying to give a definite number for her flight speed, give a relative, qualitative value. Say something like, "She's a bit faster than x but slower than y". We really don't have a firm way of comparing units from our world to Equestria, so it's probably better not to try.
And just a quick note about Rainbow Dash. Your character can't have a pre-existing relationship with her, but I suppose looking up to her would be acceptable.
If you have any questions, feel free to drop me a PM, otherwise I'll be looking forward to any updates you might make!