Vardan woman

 was a member of the Vardan invasion fleet that attacked the asteroid Grace Alone. She was the only Vardan who chose to stay behind instead of returning back to their home planet after being unravelled by the First Doctor. To avoid this unraveling, she assumed a see-through humanoid female form. She attempted to revenge the failed invasion by killing the Doctor, Steven Taylor and Oliver Harper, unraveling herself in the process. But Oliver sacrificed himself to save his friends. (AUDIO: The First Wave)

However, some remnants of the Vardan planted themselves in the Doctor and Steven. After regaining consciousness, she manipulated Steven after his abdication from being the king into building a radio telescope to bring the Doctor to the planet. While Steven believed that he was saving the Doctor, the close proximity of the two carriers of the Vardan's energies was intended to facilitate her reconstitution. Still consumed by vengeance, the Vardan wreaked havoc on the planet and planned to kill the Doctor, Steven and Sida, Steven's granddaughter and current ruler of the planet. Sida lured her into the computer systems of the laboratory formerly occupied by the copy of the Doctor's mind. The copy activated itself and unraveled the Vardan much in the same manner as the Doctor himself defeated the Vardans on Grace Alone. (AUDIO: The Locked Room)