Which of the following, somewhat nefarious looking characters, is out of place, and why?

The answer is below the pictures, in white: run your cursor of the text and hold, to see it.

Harry Furstenburg is the only one of the 6 mathematicians not to have been awarded the Field’s Medal

It’s common knowledge that mathematicians are scruffy, old, bald, bearded men, who could double for axe murderers, right?
So which of the following people are mathematicians?

[The answer is written below the last images, in white. To see the answer highlight the hidden text with your cursor.]

All of them!