Blind and connected to the Game Station feeds at the age of five by dozens of cables, the Controller acted as the eyes and ears of the Daleks. Aided by programmers, she constantly monitored the transmissions that flowed through her. Essentially, she served a function similar to the Editor's for the Jagrafess a century before, who also had occupied Floor 500 of the satellite, though the Controller had less say over her actions.
Nevertheless, the Controller loathed and plotted against the Daleks, ensuring that the Ninth Doctor and his companions would appear in game shows on the Game Station, which the Daleks ignored. She waited until a rare solar flare temporarily disrupted communications between the Dalek fleet and her.
The Daleks could not monitor her thoughts, letting her warn the Ninth Doctor of the enemy "waiting in the dark". She then passed on to him the co-ordinates of Rose Tyler, enabling him to rescue her, though she did not at that point reveal to the Doctor and his allies the identity of her "masters".
The Controller told the Doctor part of the coordinates before she was transmatted to the main Dalek saucer, where the Daleks exterminated her.