When a user has not chosen to provide profile details, the boilerplate text is:
Apparently, this user prefers to keep an air of mystery about them.
That reads poorly, as noted by this answer on SE EL&U and some of the other answers to the question, Is there a correct gender-neutral singular pronoun ("his" vs. "her" vs. "their")?
There are alternative ways to word the sentence that would be better. Consider the following suggestions:
Apparently, this user prefers to keep an air of mystery.
Apparently, this user prefers to keep an air of mystery about themselves.