What's the word to refer to someone about whom you can never make assumptions?

It's a pleasing challenge because the very notion of a character trait suggests you are trying to qualify or define them in a certain way.

I would say "enigma": in its everyday sense, this suggests a puzzling entity which seems unsolvable, impossible to be understood, prone to always surprise you.

PS In a theological context, an enigma is solvable, compared to the unsolvable "mystery/mysterium"