1.) People who refuse to give up.
Who will try more than one avenue to their goal, more than two, more than ten if that's what it takes. The ones who move over, under, around and through the obstacles.
2.) People who have no other love in their heart.
The ones who have no backup plan or fallback career. They have to make it work or they starve.
3.) Practical people.
The ones who set aside time to work toward their goal and don't let the initial fire of enthusiasm burn out after the first few weeks of a project. The ones who know that it takes a while for a project to gain momentum, but that once it starts gaining traction, the growth is exponential.
4.) People who believe in themselves.
There will be times in your career when the only person who believes in you is you. If that alone can't sustain you through the tough times, then you should find another field.
5.) People who aren't too proud to work a day job. Not everyone gets a fellowship or grant to fund their projects; some people have to subsidize themselves. There is absolutely nothing wrong with paying your bills while you pay your dues.