Gemini Shadow was the Mirror universe incarnation of her Federation counterpart.
Gemini was born as one of the first test cases of a cloning program begun by Starfleet Medical and the Equestrian Solar Empire. Conceived as a humanoid, female clone of her father Orion, Gem shares her father's vast intellect, unique problem solving skills, and complete lack of moral compass with the advantage of having, as her father called it, "a woman's touch."
Gem and her fellow clones were taught at home by their parents. Magically speaking, she is capable of using traditional pony magic, though through the medium of her hands rather than a horn. This ability was discovered when she murdered the town clockmaker in cold blood for speaking out against Empress Celestia.
At 18, she emigrated to Earth and started school at Starfleet Academy as a cadet. After graduation, Gem and her fellow Equestrian humans joined the crew of the ISS Icarus, where she served as First Officer for two years as part of a shakedown cruise. This abruptly came to an end when, on 17 June 2403, she committed mutiny against her commanding officer, Clayton Forrester, for refusing to follow the trail of a lost Federation ship. She decapitated him and sent his head rolling down the floor of the bridge, announcing herself as the new captain shortly thereafter.
Later that same day, the Icarus came across the lost Federation vessel, the Galaxy-class USS Axanar (NCC-72614). After threatening the captain, she and her crew boarded the ship and killed the Axanar's crew, leaving no survivors. After a celebratory round of flogging with her slave, Dustin Hoofer, she was given the order by Empress Celestia to recreate an ion storm in Halkan space, in the hopes of opening a rift into the Federation's universe.
The attempt succeeded, and on 7 September 2403, an away team mirror Endeavour swapped places with their Federation counterparts, causing chaos wherever they went. After a while they returned to their own universe, and Gem spent several months devising a plan to return.
On 20 August 2404, Gem successfully returned to the Federation universe via device of her own creation, a dimensional ring worn on her finger. There, she attempted to murder her Federation counterpart, only to fail in the attempt. After returning to her own universe one more time, she was shot in the head by her sister, killing her instantly. Her body was then set aflame and used as kindling for a wiener roast.