I took a Doctor Who companion personality quiz on the BBC America website.
Of the 6 modern companions I ended up to be Martha.
"While occasionally lead by your heart, you have an enormously practical head on your shoulders, and are extraordinarily loyal, which means you can be relied upon to fulfill any task to the letter, no matter how personally inconvenient it may be. Guns, bandages, science, amorous playwrights from the past, there's little you can't handle."
Click here to give it a try.