Here is a link to the long story series she was in.
Emerald is described as Twilight's mirror image, with emerald color body, and glasses.
Doctor Emerald Sparkle was a student under Queen Gaia, Celestia's mother. She was a close friend of Celestia and Luna. She became a scientist who invented nuclear energy, however, Queen Gaia developed radiation sickness, and died.Celestia was furious and when she became an alicorn princess, she destroyed and banned most technology. Emerald hid in her weapons facility, EPMB for the entirety of Luna's banishment. Luna hired her to keep watch over Equestria for her secretly, which she did. She invented combots, spritebots, and combat modeled spritebots. She had made a machine to keep herself alive after being attacked by Surprise, and she was able to control the facility from the mainframe.
She spied on Equestria using invisible spritebots and secretly developed weapons in her spare time. She broke the deal with Luna, and attempted to seize control of Equestria, sending an army of combat-model spritebots and combots through Ponyville. They were destroyed before getting to Canterlot during the Battle for Ponyville.
Twilight rushed to EPMB after the Battle For Ponyville and threatened to kill Emerald. Emerald said she had a fail-safe, which would launch nuclear warheads upon her death. Twilight decided to disconnect her from the mainframe, to which Emerald expressed fear. She said she'd lied about the fail-safe, Twilight ignored her. She was disconnected from the main-frame, rendering her helpless in stasis, stripping her of control of the facility. She lost her life support as well, it is assumed she died in stasis, the disconnection would cause a vast amount of brain damage.
Minutes before death, she left a note, found by the Cutie Mark Crusaders. Originally saying to leave EPMB as is, but quickly took it back, and to use the technology as they pleased. The CMC used the technology, and developed a business that slowly brought Equestria into the modern age, called FutureTec.