The Warrior, a self proclaimed god of war, who wonders the universe and time fighting in different wars. I don't really try to change anything, just do it for the kicks. Also I have around a dozen companions, all exceptional soldiers and fighters drawn from different ages, there's a moderately high turnover rate because all but the best die (and even they die eventually). They're free to leave whenever they want but drawn by a sense of adventure.
The Tardis usually is in the appearance of a WW2 German U-boat (because Das Boot, was awesome).