Katarina

Katarina was a companion for a very short time. She may also have been the first companion to die while travelling with the Doctor. Before meeting him, Katarina was barely more than a slave. A Trojan, she was the handmaiden of Cassandra, Princess of Troy.

Biography
Cassandra sent Katarina to spy on the travellers, and in particular Vicki - known by Cassandra as Cressida. Katarina was sent to gather evidence that Cressida was a greek spy. She befriended Vicki/Cressida, who sent her to help the Doctor get the wounded Steven Taylor back to the TARDIS. Once there, Katarina tended to Steven's wounds, and joined Steven and the Doctor on their travels.

Katarina came from a simpler time. She was from an era which believed in prophecies, auguries, heroic myths, and Gods. She never understood where she was, or what was happening to her. Katarina believed that the Doctor was a God, who was taking her on a journey beyond, in his temple. Despite the Doctor's protestations to the contrary, she remained set in this belief. She firmly believed that this was a devine journey to the 'place of perfection'.

Always loyal to her new 'master', Katarina used the knowledge she had gained in her short time with the travellers to open the airlock door on Mavic Chen's spaceship, 'Spar', to sacrifice herself and save her friends. She was certain that her death had been foretold, and that she was doomed to die. Her travels seem to fit into the prophecy. Her self-sacrifice was not, however, because she believed that was the way she was supposed to die. Katarina sacrificed herself to prevent Kirksen from blackmailing the Doctor into returning the ship to Kembel, and the Daleks.

Quotes
"Can we have reached the Place of Perfection so soon?"

Doctor: "She wanted to save our lives, and perhaps the lives of all other beings of the solar system. I hope she found perfection."

The Daleks' Master Plan