K-9 Mark I was built by Professor Marius to accompany and assist him in his work at the Bi-Al Foundation. He served as the template for all future versions of K-9.
Profile
Biography
He joined the Doctor and Leela (DW: The Invisible Enemy), but left with Leela when she decided to remain on Gallifrey with her new lover, Andred. (DW: The Invasion of Time) Later, it is believed that the Time Lords assigned K-9 the command of a space vessel called K-NEL and gave him occasional assignments. (The Adventures of K9)
It is unknown if he survived the Time War as Gallifrey was destroyed, though his mistress, Leela, did (CC: The Catalyst)
Notes
K9 Mark 1 is rumoured to return in the first episode of K-9 (TV series). Bob Baker has said that the K9 model that appears in the early part of the first episode is "identical to the one Leela had in Galifrey (sic)". This means K9 Mark 1 might have survived the Time War. In the pilot episode, he will appear as last seen in Doctor Who but will before expiring, insist his companion initiate his "regeneration" sequence, from which a new and more futuristic form of K-9 Mark I will emerge.