The sign of the massive, long-charge justu-bomb being worked into her body is her (for her) frustrating inability to do anything with seals.
Don't have all her faults and weaknesses caused by something. Everyone has them, no one is good at everything, don't make excuses for being bad at something. Just be bad at it.
Likewise don't make her unusually good for her rank, remember she's been a genin for a couple of weeks and has no field or team experience. It should show. The rest of the team (sans Aoko who's ANBU) have all made it to the level where they've been in a chuunin exam before and have at least 2 years experience.
And the general rule of making a character's past:
Have fun, make it interesting, make sure it makes sense, know what a
Sue is so you don't fall into it.
This may be a more personal thing but how my characters (especially in NQ) act is based a lot on their past. Their personalities are some what inbuilt, but are changed and morphed by what happened to them. Their past should explain why they are the way they are instead of being shoehorned in. It's about working to make the character deeper which in turn makes this interesting. Rather than picking and choosing irrelevant traits and life events in the hope of making it interesting.
I spoke about this with ewuvi once and she does the opposite. Her characters are driven by their goals and desired future, while mine are driven by their pasts.
You have to find what works for you.
One thing which I know is personal, for God's sake have your know your timeline and be sure it makes sense.