She first appeared in Action Comics #26 (May 1940), in the feature "Tex Thompson".
[1] Miss X aided Tex Thompson and Bob Daley after the third member of their team, Gargantua T. Potts, left to become a cook in the French Army.
[2] She wore no costume, and her sole disguise consisted of a pair of dark sunglasses.
In the final panel of Action Comics #29, he confided to his partner that he thought he had figured out who she was.
Miss X made her final appearance in Action Comics #43 (Dec. 1941).