Same word as x:

Jane/John Doe, X, bearding, incognito, innominate, nameless, pseudo, pseudonymous, secret, so and so, such and such, unacknowledged, unattested, unavowed, unclaimed, uncredited, undesignated, undisclosed, unidentified, unnamed, unsigned, unspecified, what's his/her name, whatchamacallit, whatsis, you know who