enigma
/əˈnɪɡmə/noun
A person or thing that is mysterious and very hard to understand or explain.
Example sentences
- “Even after working with him for years, my quiet colleague remains an enigma to me.”
- “How the ancient builders moved such huge stones is still an enigma.”
Churchill's 'a riddle wrapped in a mystery inside an enigma' keeps the word famous. Applied to a person it suggests fascinating unreadability.