A traveller who's the last of a noble race known as Time Lords. Sometimes he has companions, but they never stay along forever. They age and eventually die, and he has to move on.
He's the most fantastically good man throughout all of time and space, and yet he gets so little.
At the end of the day, he's just an oft lonely man who travels around the universe in a little blue box. And that's what makes him so brilliant.
"Who are you?"
"I'm the Doctor."
"The Doctor? Doctor who?"