Anya Kingdom

Anya Kingdom was a member of the Space Security Service who posed as a police officer called "Ann Kelso" from 1970s Earth, and a companion of the Fourth Doctor.

Biography
Ann first encountered the Fourth Doctor after Jimmy Lynch's gang attacked Tony's café. She then hid Tony in her flat and helped the Doctor to defeat the Sinestrans. She asked to join him on his travels and he accepted. (AUDIO: The Sinestran Kill)

In reality, "Ann Kelso" was a fake identity created for Space Security Service officer Anya Kingdom, who had allegedly been sent back to protect Tony after he called for help. She had started travelling with the Doctor as she subconsciously recognized that the TARDIS would be the perfect way to find the rest of the Syndicate. While the 'Ann Kelso' personality was mostly in control, Anya would take control to deal with threats and potential enemies in a more ruthless manner than Ann might have preferred.

Once Ann's true identity was revealed, the Doctor determined that she was actually an agent of Zaal, the head of the SSS and Earth's representative in the Syndicate, who had sent Anya to use the Doctor to eliminate his associates in the Syndicate so that he could take power himself without them realising the truth. Fortunately, the Doctor was able to convince Anya that she had been just as brainwashed as Zaal's other agents and helped her break her conditioning in time to trick Zaal into a confession.

Although Zaal's control over her was broken, the Doctor declined Anya's request to continue travelling with him, as he had genuinely liked Ann Kelso and found it hard to look at Anya as anything more than the reason Ann wasn't there any more. Despite her regret, Anya accepted his decision and decided to focus on the new task of rebuilding civilization after Zaal's illusions had been removed. (AUDIO: The Perfect Prisoners)

Behind the scenes
The character of Ann Kelso was written specifically for Jane Slavin due to her extensive work on the Fourth Doctor Adventures and her close friendship with Tom Baker.