Koschei (Rebel Rebel)

Chris Cwej and Frey liberated a "Koschei" with green eyes and a dark beard from the Tower.

Koschei was a name used by the Master prior to taking on the latter title. (PROSE: The Dark Path) Koschei had been locked there by the Superiors as punishment for "a silly prank gone wrong", in the bathroom of a bar on an underlevel of the Tower. After an argument, the friend escaped, leaving Koschei behind. He was later found by Cwej and Frey during the V-Time era. Frey was dying because, as an Ephem, she needed regular infusions of Superior biodata: Koschei offered to give her his own and lead them out of the dungeon, on the condition that Cwej promise to listen to Koschei's words on a future date. Later, when Cwej was using the Tree of Life to undo the Cwejen Uprising, Koschei spoke through Frey telepathically, distracting Cwej; as a result, only half the events of the Uprising were undone, resulting in an unstable new status quo. (PROSE: Rebel Rebel)

By the time he met the Second Doctor on what would be his final adventure under that name, Koschei had regenerated into a form identical (PROSE: The Dark Path) to that he wore as the Master in his rivalry with the Third Doctor, (TV: Terror of the Autons, etc.) although his encounter with the Second Doctor seemed to end in this body's apparent death and the loss of several regenerations to the Darkheart, suggesting that they may not have technically been the same incarnation. (PROSE: The Dark Path)