played by John Leeson (voice)
Also played by David Brierly (Season 17)
K-9 joined the TARDIS when his original owner, Professor Marius, was unable to bring him back to Earth due to luggage limits. K-9 was a robotic dog which functioned as a mobile computer/defence system.
K-9 Mk I stayed with Leela on Gallifrey, the Doctor produced a K-9 Mk II, which remained in E-Space with Romana.
K-9 Mk III was a gift from the Doctor to Sarah Jane Smith.
K-9 Mk IV was also a gift from the Doctor to Sarah Jane Smith when K-9 Mk III was destroyed in an explosion.