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

I want a word to describe Macavity, the Mystery Cat to denote that you could never assume anything about him. If you did, he was sure to prove you wrong. Can you help me by suggesting an appropriate word? Thanks in advance...

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"