"Apparently, this user prefers to keep an air of mystery about them" I happened to click on a user's MO profile. Under his clearly set-out name was the legend "Apparently, this user prefers to keep an air of mystery about them". On the same page is his photograph and a link to his academic page detailing his affiliation and interests and a considerable list of publications and other professional achievements. What would he need to do to lose this "mysterious" appellation? (I haven't done any research about who is or is not so described, beyond noting that I am spared the comment in my own profile even though I have been far less forthcoming than the said user.)

According to this meta stackexchange post, it just means that the "About me" section was left blank. (If you are looking at your own profile page and see the message "Your _about me_ is currently blank", then it means that others looking at your profile get the air-of-mystery message.)