Donna is immediately like a sister to the Doctor
1. She does find all the crazy, dangerous stuff wonderful/fun
2. She gives the Dr. a good kick whenever he needs one: insulting him, telling him to shut up, and not letting him get TOO full of himself)
But the best part is that Donna’s character is not reliant on the dr.
1. She seeks out and continues on without him
2. She isn’t in love with him
3. She isn’t nice! (that isn’t to say she’s mean) but she doesn’t quibble, she isn’t saccharine, she’s real and she wants the REAL answers