Kirksen was a criminal stranded on Desperus.
Biography
Kirksen boarded the Spar when it was forced to land there, took Katarina hostage, and tried to blackmail the Doctor to take the ship to Kembel. Bret Vyon refused to obey the criminal, while Steven Taylor wanted to save his friend, and the two argued over which course of action to take. Katarina rendered the argument moot when she opened the airlock door, killing Kirksen and sacrificing herself. (DW: The Daleks' Master Plan)