Cass Fermazzi

Cass was the sole remaining pilot of a space gunship. She died when the ship crash-landed on the planet Karn, committing suicide by rejecting the aid of her rescuer.

Cass had already teleported the rest of the crew off the ship when she encountered the Eighth Doctor, whose TARDIS had just materialised onboard. Recognising the Doctor as a Time Lord, Cass chose death over involvement with the Last Great Time War, despite the Doctor's protestations that he wasn't part of the war. The Doctor refused to leave her side even when it meant he would also be killed in the crash.

Her death inspired the Doctor to finally take part in the war. He claimed her bandolier as a reminder of what he would be fighting for and chose to regenerate into a warrior after the Sisterhood of Karn revived him long enough to select what his next incarnation would be like through their elevated knowledge of Time Lord science. (TV: The Night of the Doctor)