Beth Stokes

 was the leader of a Valkyrie unit. When she led her unit to a Scientific Facility Rawke 279, she wished it to be evacuated after her crew were treated for injuries by the Sistermatic. She blamed herself when the Daleks appeared to have followed her. This drove her mad, to the point where she deactivated the defence grid, to let the Daleks in. Her colony was invaded in her youth. Her older sister Becka and she got taken into a Dalek work camp, where Becka dies. Her mind returns to her when Ace reveals that it wasn't her fault for the Daleks being there. She chooses to stay behind so that the Black Daleks selfdestruction isn't tamperd by the Kiseibya. (BFA: Enemy of the Daleks)