a nut
Examples
A fanatic is a nut who has something to believe in.
When you hire me, you hire a nut who is going to work 24 hours a day for you and never, ever burn his audience.
I think the average person thinks I'm a nut and I deserve whatever happens to me.
Even a blind squirrel finds a nut once in a while.
A nut is good if broken, a women if beaten.
She's a nut.