I know Ben 10 isn't an anime, but the girl is an anime character called Shana from Shakugan no Shana. Don’t worry because I’m only using the anime title and pictures of her, not the story, so you won't get confused. The story is set in Ben 10 Ultimate Alien Force. By the way Ben 10’s story is going to get a bit twisted, as in rearranged. So in Ultimate Alien Force, Ben is 16 years old.
Name: Shana Shakugan also known as the Fire Princess
Status: Plumber (she fixes any problems aliens cause on Earth)
Crush: Ben Tennyson
Your name is Shana Shakugan, you are from another dimension of the same earth, but with different inhabitants. You were the princess in that world and the people in your world had the power of fire, but the people were still humans not aliens. Just humans born with a special power. Your people didn’t know about the other Earth dimension, other than the Royal family, which was you and your parents. Your world was constantly disturbed by aliens from both your dimension and the other dimension. One day a powerful alien came and the only way to stop would cause your dimension to collapse and be destroyed forever. Your parents knew if they didn’t sacrifice their dimension the other dimension will also be destroyed by the alien and the human race would be eradicated. Before they destroyed the alien they sent you to the other dimension to help protect it. You were 14 years old when all this happened, so your parents died, your world died and your now 16 years old. You’ve lived in this Earth dimension for 2 years as a plumber, fixing any problems aliens cause on this Earth. You knew this dimension just like your old one, because it is identical, the inhabitants are just different and you have to be cautious when using your fire ability. You don’t want to freak out the humans of this dimension. You owned safe houses all over the world, bought with immense money you had been given when you left your dimension. Of course the houses were under your parents’ names, you had created fake identities for them in this dimension since you are under age to own houses. You could drive anything, cars, trucks, tanks, ships, boats etc. You could also fly anything, planes, helicopters, etc. So you were set for life, you use a katana (a medium-length Japanese samurai sword) and your power of fire when you fight aliens, and just your sword for regular humans which are normally thieves, muggers and so on. You are rich and you had to attend high school. It’s part of your cover since you don’t really exist in this dimension. You created a fake identity for yourself, but it’s your real name. You go to high school, but always transferring. Trying to find one you would blend in and like the work and so on.
Julie, the one in the cartoon who is actually suppose to end up as Ben's girlfriend does not exist in my story. =)