Hegelia

 was an ArcHivist, the most respected researchers from the ArcHive. She and Graduand Jolarr time travelled back to study the aftermath of the Cybermen's occupation of Agora. However, Hegelia altered the coordinates so they arrived during the last year of the occupation. Her fascination with the Cybermen so consumed her that she wanted to undergo Cyber-conversion, to completely understand them. They refused, so she used Overseer Madrox to run the Cyber-conversion equipment to achieve her goal. After the process was finished, she killed Madrox. (PROSE: Killing Ground)

Behind the scenes
Hegelia and the ArcHive are taken from the non-narrative book, Doctor Who: Cybermen, by David Banks.