She's so cool "her name is Martha and she's nobody's collateral damage, G.I. Jane." Cool. And I like that the Doctor is so busy running around and figuring out the whole thing with his daughter and Donna's the one who's thinking about the numbers and figures it out.
She's like me (well, I don't mean that literally but I can identify with her, which I couldn't with Rose for the life of me!): unapologetically frumpy and excitable and realistic, and level headed, and does what she needs to do even when she's scared out of her mind, and then proceeds to have a loud nervous breakdown.
Go, Donna!