Twenty

 was a programmed human like Zoe Heriot. The Second Doctor asked his help in finding Zoe Heriot and helped solve the equations. He fought with Zoe about the technical routines. He helped Blavatsky to escape but couldn't process Lorne's death. He tried to flirt with Zoe. She used his abilities to contact Jamie via Nostic. This damaged his sensors. He sacrificed himself to find the Corvus prism to stop help the Doctor stop it. (AUDIO: The Wreck of the World)