The mirror-universe Jadzia Dax is a Trill mercenary fighting with the Terran rebellion and the mistress of its leader, the mirror Benjamin Sisko, whom the prime-universe Sisko must impersonate.
In the mirror universe Jadzia is a tough, plain-spoken fighter who has thrown in her lot with the Terran rebels. She is the lover of the mirror Benjamin Sisko and one of his closest lieutenants.
She first appears in Through the Looking Glass. Believing that her captain has returned from the dead, she welcomes the prime-universe Sisko rather warmly, leaving him to keep up his disguise in awkward circumstances.
She returns in Shattered Mirror, by which time she has learned of the deception and greets Sisko with a slap. She takes part in the battle to hold Terok Nor against the Alliance fleet.