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K-9 was the first - and until Kamelion, the only - robotic companion to travel with the Doctor.
Biography
There were three K9s. K9 Mark I was built by Professor Marius to accompany and assist him in his work at the Bi-Al Foundation. This K9 joined the Doctor and Leela, ("The Invisible Enemy"), but left with Leela when she decided to remain on Gallifrey with her new lover, Andred ("The Invasion of Time").
A K9 (Mark II) ("The Invasion of Time"), constructed by persons unknown and more or less identical to the original. He accompanied the Doctor and his new friend on their quest to locate the segments of the Key to Time ("The Ribos Operation"), however the swampy terrain of Delta Magna's third moon made it impossible for him accompany them there. At the conclusion of the quest, the Shadow turned K-9 to his evil will, during which time he assumed a sinister personality ("The Armageddon Factor").
K-9 lost his voice ("Destiny of the Daleks"). During the Doctor's next adventure ("City of Death") he remained patiently in the TARDIS, implying, perhaps that his voice had not yet returned. Subsequently ("The Creature from the Pit"), his voice sounded quite different and remained so for a number of adventures, however by the time we saw him vacationing with the Doctor and Romana in Brighton ("The Leisure Hive") he once more sounded like his familiar self.
Damaged by the time winds, K-9 Mark II could not cross back into N-Space with the Doctor, so decided to stay behind at the Gateway with Romana, to help her free the remaining Tharils from slavery ("Warriors Gate").
The Doctor (presumbably) at some point built K9 Mark III, which he left on Earth as a present for another companion, Sarah Jane Smith ("K9 and Company"). At a later date, K-9 Mark III still lived with Sarah Jane, but, by chance, would not accompany her to Gallifrey ("The Five Doctors").