Q (also refered to as the "Female Q" and "Lady Q") was a member of the Q Continuum who, during the Q Civil War in 2373, opposed changes of the kind advocated by her former boyfriend, Q. During the war, she recruited Doctor Selar of the USS Enterprise-D to serve as both a medic, and to provide a logical, objective viewpoint of the conflict. To end the war, Q agreed to conceive a child with Q. Q gave birth shortly after the conception, with Selar serving as midwife.
She later appeared on the USS Enterprise-E in 2374, when Betazoid scientist Lem Faal's experiments at the Galactic barrier threatened to release the entity known as 0 from his confinement.
In 2380, Lady Q appeared to Admiral Kathryn Janeway to warn her away from personally investigating a seemingly dead massive Borg cube. Janeway ignored her warning and Lady Q stood witness as Janeway was assimilated and transformed into the new Borg Queen.
With the destruction of the cube and the apparent death of Janeway, Lady Q took Janeway away from the scene to face some unknown "destiny". She served as a direct participant in the events of the mission by serving aboard the USS Enterprise-E, in the guise of Jon Stephens.