who; what kind of person
Used to ask about someone's identity, often with a nuance of suspicion or curiosity about their true nature or background. Common in questions like 何者だ？ or 何者なんだ？
あの男は一体何者なんだ？
Who on earth is that man?
Nobody knows who he is.
Who on earth is that guy? He suddenly showed up and seems suspicious.
My car was scratched by someone unknown in the middle of the night.
I feel anxious about being unable to become anything or anyone in particular.