The Terran counterpart of Sylvia Tilly, a feared officer nicknamed Captain Killy who commands the ISS Discovery and whom the prime Tilly must impersonate when her ship is stranded in the mirror universe.
Captain Killy is introduced by reputation in Despite Yourself, when the crew learn that their awkward cadet is, in this universe, a notorious commander who took her ship by murdering her predecessor. The prime Tilly plays the part over the viewscreen with growing enthusiasm.
The real Killy is not seen in the first season. Her ship changed places with the USS Discovery, and The War Without, the War Within reports that it was destroyed by the Klingons soon after arriving.
She finally appears in Terra Firma, when Philippa Georgiou is sent back into her own past. There Killy is a cruel and loyal officer of the Emperor, and Mary Wiseman plays her with relish.