Flaws:
Temper. Lots of swearing, shouting, stammering nonsense, crying when angry.
Insecurities:Eveyone probably thinks they're better than me. Everything I do is wrong. I'm not pretty enough/strong enough
Swearing.
Assuming the worst/pesimistic
If you're trying to assess reactions to this character's personality, you may wish to write up a short scenario and have her interact in it. Generally, when I'm reading a story or watching a show, I like to see the character's traits surface one by one or as a whole. This is what keeps me from getting bored as I continue to watch because a good writer knows how to use subtlty to slowly paint the picture of this person's inner being.
That said, here are my initial reactions to the post as a whole.
Flaws:
Temper. Lots of swearing, shouting, stammering nonsense, crying when angry.
Insecurities:Eveyone probably thinks they're better than me. Everything I do is wrong. I'm not pretty enough/strong enough
Swearing.
Assuming the worst/pesimistic
So, this is a potential seventh Animorph. First off, the flaws are actually very optimistic to me as it means this is going to make for some very interesting character development. I can all ready see the problems this character will have with the Animorphs.
For one thing, Jake may be the leader, but Rachel is the Queen Bee. And Queen bees don't like competition, which is what this girl screams of.
You've listed "swearing" twice as a flaw. As you're British, I can only assume that the words you're thinking of wouldn't be readily considered cussing by Americans. (IE, I got away with saying "bloody" a lot since most teachers didn't register it as me cussing) Because I don't see what we Americans refer to as the "F-bomb" being terribly appropriate in this forum. And if you're only implying that she cusses every five seconds, that's going to get tedious and boring to read very quickly.
The stammering and babbling you mention is very interesting. Mostly because I have to ask myself, how is this girl going to get a hold of the morphing power in the first place and how useful is she going to be? Will the mere sight of Visser Three's troops cause her to lose composure?
Remember, David was a pretty smart kid. Even he lost it and by comparrison to his girl, he was at least partially stable which is how he fooled the other Animorphs. Is she going to get to the "Don't hurt me, I'll morph out" stage or are the better, wiser Animorphs going to see all of the signs and tell her to sod off?
As for a move to the States.
There are quite a few British people living in or around the Boston, Mass area. In my hometown of Bennington, Vermont we had a British librarian for a few years and there was a teacher in my school who taught computer programming. (He was an idiot, after I dropped his class he tried to get me in trouble for "skipping", even though I was no longer attending his course. But that's neither here nor there.)
So whatever the overall reason is or the general feeling about the US, there are plenty of legitimate reasons for your character and her family to move to the States. Employment being the major one.
These are just some things to think about as you're writing the story.
