Captain Sylvia Tilly (simply known as Sylvia Tilly) is an antagonist who was introduced in the Star Trek: Discovery episode "Despite Yourself".
She was portrayed by Mary Wiseman.
Thus far Tilly - aside from an image in a personnel file - has not actually appeared in licensed media. The image of Tilly from her file was portrayed by Mary Wiseman, who portrays her primary universe counterpart. Unlike her counterpart, Tilly's hair was colored blonde and she wore it straight.
An officer in the Imperial Starfleet of the Terran Empire, Tilly began her career at the Imperial Starfleet Academy, where she graduated with distinction. She also participated in actions that led to Tilly being known as The Slayer of Sorna Prime and The Witch of Wurna Minor.
Tilly later became the Captain of the Crossfield class starship ISS Discovery after murdering the ship's previous Captain. Her actions as Captain further led to her becoming known as Captain Killy. Tilly was one of Emperor Philippa Georgiou's favorite Captains. Tilly worked with her Emperor to subjugate Betazed and destroy Mintaka III.
During her career Tilly was decorated for valor, for having 50 kills, and being a Master of Poisons. Her prime universe counterpart was greatly disturbed over Tilly. Her counterpart said Tilly was nothing like her, that she was a twisted version of everything she aspired to, and that she would have nightmares about her mirror counterpart.
In late 2256, she and the crew of the Discovery switched places with their prime universe counterparts. Upon arrival in the prime universe her ship was destroyed by the Klingons, resulting in the death of Tilly and everyone else on board the ISS Discovery.