- "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."
- —Sylvia Tilly to Michael Burnham, 2256[src]
In the mirror universe, Sylvia Tilly was a female Terran, a subject of the Terran Empire serving as officer in Imperial Starfleet in the 23rd century. In the 2250s decade, Captain Tilly commanded the ISS Discovery. (DSC episode: "Despite Yourself")
In her final year, Tilly was part of the Class of 2256. When their instructor, Captain Anton Schaefer, was killed, Tilly murdered Schaefer's favorite student with a spoon, and assumed command. (STO - "Mirror of Discovery" missions: "Para Pacem", "Pahvo Dissension")
- In the primary universe, Captain Schaefer died on a cadet training cruise. His acting XO inherited command, and Tilly acted as chief engineer. (STO - "Age of Discovery" mission: "Routine Maneuvers")
Later, Tilly served aboard the Crossfield-class science vessel ISS Discovery. When her captain was recovering from Crestian flu, she murdered him and assumed command. (DSC episode: "Despite Yourself")
Tilly distinguished herelf at locations like Quam, Elpha and Purmata VI. Epitaphs included "the Slayer of Sorna Prime", "the witch of Wurna Minor", and "Captain Killy". Tilly earned medals for Valor, Master of Poisons, and 50 Kills. (DSC episode: "Despite Yourself")
In December 2256, Tilly and the Discovery were ordered to Pahvo. Due to the dangerous nature of the planet, Tilly presumed somebody wanted to "retire" her permanently by sending her on a suicide mission. At Pahvo, the Discovery came under attack by a Klingon fleet led by Captain Kobax. (STO - "Mirror of Discovery" mission: "Para Pacem")
Fortunately, it was at this point Gabriel Lorca of the USS Discovery from the primary universe used the ship's spore drive to translocate into his native mirror universe. Pretending to be his primary counterpart, Lorca let his crew discover the nature of the Terran Empire. Armed with that intelligence, Ensign Sylvia Tilly masqueraded as Captain Tilly. (DSC episode: "Despite Yourself")
Meanwhile, the ISS Discovery and Kobax's fleet appeared at Pahvo in the primary universe, in the year 2410. Tilly requested asylum from an approaching Alpha Quadrant Alliance starship. Chief Rathmon then staged a mutiny against her for making a deal with aliens. An AQA away team beamed aboard at Tilly's request, and cleared out Klingon warriors and Rathmon's mutineers from the lower decks, liberting security chief Patel from an agonizer booth. Tilly remained in contact with the team through communications. After they prevented a warp core breach, she invited them to the bridge, just in time to defeat a landing party by Klingon captain N'tarn.
The Allied away team returned to their ship when Kobax and his fleet returned. During the battle, Tilly directed Discovery closer to Pahvo and beamed down, leaving Patel in command. The captain used agonizer technology to corrupt the Pahvan crystals, destroying a trio of M'Chla-class birds-of-prey with a powerful beam, and damaging the Allied vessel severely. Tilly spoke with the Allies and Chief Researcher Somat, advising them that she considered sparing the Allied ship as fulfilling a debt for saving Discovery from the Klingons. When she threatened them, the ship left. (STO - "Mirror of Discovery" mission: "Para Pacem")
Tilly and her forces raided the Vulcan research camp on Pahvo. She tried to bring reinforcements from the Terran Empire of 2256, which ended in the wreckage of a Styx-class dreadnought arriving in orbit.
Somat developed technology to purify the Pahvan crystals, and accompanied the AQA back to Pahvo. The ship defeated Klingon ships near the dreadnought's wreckage and rescued Imperial officers from there before sending an away team to the planet. Somat accompanied the team, and they treated the Vulcan scientists before fighting Tilly's troops. Tilly and loyal commanders fought with the AQA but were defeated. Tilly used the giant Pahvan crystal to transport herself to unknown whereabouts. With Discovery subdued, Patel agreed to return to her timeframe. Somat and the Allied leadership were concerned about Tilly, who remained at large. (STO - "Mirror of Discovery" mission: "Illusion of Communication")
Discovery ended up in its own time, but the primary universe, and was destroyed by Klingons. Admiral Katrina Cornwell saw the wreckage with her own eyes. When the USS Discovery returned nine months later in 2257, Starfleet concluded Tilly and her Imperial crew had died. (DSC episode: "The War Without, The War Within")
Meanwhile, wreckage of the ISS Charon rained on Pahvo. Tilly worked with nine loyal captains - fellow survivors of the Class of 2256 - to corrupt Pahvo again. Her captains were killed and her plans foiled once more by an AQA party. (STO - "Mirror of Discovery" mission: "Pahvo Dissension")
In 2380 of a timeline wrecked by the temporal anomaly crisis, Tilly appeared without Discovery and resumed her scheming ways. Known as Captain Killy, she partnered with a Tieran-possessed Kes and the con artist Ardra. The trio seized the Beta Penthe system and freed the prisoners of Rura Penthe. In a public announcement, Killy stated her goal of taking the entire Alpha Quadrant, starting with the Federation and Klingon Empire.
Versions of Sylvia Tilly, Kes plus personalities from the Klingons and Terran Empire worked with the Federation in capturing Killy. Following her incarceration, Emperor Spock declared not to execute her, to avoid turning her into a martyr. Killy pronounced him a fool and she would escape eventually. Subsequently, less volatile versions of Killy could be recruited by the Federation. (ST video game: Star Trek Timelines faction event: "The Butchers of Beta Penthe")
- Sylvia Tilly (mirror) article at Memory Alpha, the wiki for canon Star Trek.
- Sylvia Tilly (mirror) article at The Star Trek Online Wiki.