The Mark I was particularly associated with and protective of Leela. Therefore, when she elected to remain on Gallifrey, he also chose to stay behind with her. (TV: The Invasion of Time) Nevertheless, he occasionally left Leela's side when he was sent on solo missions by the Time Lords. During these adventures he travelled in a spacecraft the Time Lords build especially for him called K-NEL. (PROSE: K9 and the Missing Planet, and others) Later models — or marks — of K9 were based on this one.