(coexisting on another dimensional plane)
Prior to Discovery, she had been at the Imperial Starfleet Academy. Among the locations where which she distinguished herself during her career were Quam, Elpha, and Purmata VI. She was also known as "The Slayer of Sorna Prime", "The Witch of Wurna Minor", and "Captain Killy". (DIS: "Despite Yourself")
The mirror Tilly and the ISS Discovery appear in the "Mirror of Discovery" storyline in Star Trek Online, as part of the game's 9th anniversary in January 2019; she is voiced by Mary Wiseman, who also voices her "original" counterpart. The ISS Discovery has been brought forward to the 25th century and into the "main" universe by an ion storm in orbit of Pahvo. Tilly is dealing both with Klingon raiders and a mutiny amongst her own crew, and requests the assistance of the player character. When she regains control of her ship, however, Tilly beams to the surface and begins subverting the Pahvan transmitters with agonizer energy, converting the crystal towers into powerful weapons and disrupting the Pahvans' natural harmony. The player is forced to beam to the surface to face Tilly and disable her agonizer generators; wounded and beaten, Tilly uses the transmitter as a long-range transporter, escaping to an unknown destination. The ISS Discovery is sent back to its proper timeline under the command of its former security chief, where it will eventually swap universes with the USS Discovery and then be destroyed by the Klingons.