10 Best Mirror Universe Characters In Star Trek
8. Captain Sylvia "Killy" Tilly
In the Prime Universe, Ensign Sylvia Tilly is a friendly and energetic young Starfleet Officer devoted to her crewmates. Over in the Mirror Universe, she's as antithetical a character as humanly possible and is known as "Killy" to her enemies. When Prime Tilly learned of her counterpart, she described her, saying:
"I'm nothing like her, Michael. She's terrifying, she's… she's like a twisted version of everything I've ever aspired to be. I'm gonna have nightmares about myself now."
Captain Sylvia "Killy" Tilly commanded the ISS Discovery after killing her former Captain, which is a common way of moving up in the Terran Empire. She was especially gifted at torturing prisoners for information or for fun, depending on her mood.
Though she wasn't seen much in Discovery, her story continued in Star Trek Online, where the ISS Discovery was brought forward to the 25th century and into the Prime Universe via an ion storm.
She dealt with a mutiny and some other issues before heading to the planet Pahvan to convert crystal towers into powerful weapons. When she's stopped, she transported herself to an unknown location, and the ISS Discovery is sent back to its proper timeline. Eventually, it swapped places with the USS Discovery and was destroyed.