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.

Quotes
"Can we have reached the Pleace 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 Dalek's Masterplan