Nader is kind of like the Ron Paul of the Left, in that neither of them are terribly charming people on their own, but their principled, idealist, and sometimes radical politics are charming. And with Nader, it's mostly his consumer advocate work that makes him admirable in that way.
But in his personal life, he's a chronic reader, introverted, solitary, diligent, a very deliberative thinker, and is almost too modest. I think INFJ.