This category is for people about whom there is (or was for a long period) a mystery as to their identity, immediate origins, or life.
It should not include people just because few facts are known about their lives. There are some people who live or lived very secret lives, or about whom there is not much documentation, but that is not a reason to include them in this category.
This category should include unidentified criminals only if a better category in which they fit in does not exist.
Finally, the category should not include pseudonyms, nonexistent people, characters from folklore, unidentified murder victims, or people probably created through a work of fiction (even if the author claims that the person is real, unless there is evidence outside of the work that the person exists).