Travelled with: Fourth Doctor
Series: Classic

K9 was first constructed by Professor Marius in The Invisible Enemy (Classic Series), after weight restrictions had prevented Marius from bringing his real dog from Earth into space with him. The same weight restrictions also prevented him from taking K9 back with him, so he donated him to the Fourth Doctor, where K9 then became a member of the TARDIS crew. All four K9's had a consistent personality, having a formal, polite manner with a small hint of pedantry bordering on condescension. Occasionally he would display feelings of sorry and self-regard, though often stated that he was not programmed to have emotions. K9 did not use contractions in his speech, and addressed others by titles such as "Master" or "Mistress". He would refer to the Doctor as "Doctor-Master" and would refer to younger male companions as "Young Master". Despite this, K9 did not appear to resent his subordinate status within the TARDIS crew, he would tend to side with the Doctor's companions over the Doctor himself, and would at times show a darker side, especially regarding other artificial intelligences with contempt. Despite being artificial intelligence, K9 did not engage in computer-like lateral thinking, with the exception of following Romana's instruction to "forget it" to the letter, and erasing all information he had about tennis from his memory banks in The Stones of Blood (Classic Series).

K9 playing Chess with the Fourth Doctor

Because of his vast intelligence, K9 would often bore people with facts and would not stop even if he was asked to. Occasionally he would he told to stay in the TARDIS while the Doctor and his companions went adventuring and he would argue, giving the Doctor reasons as to why he should go, showing K9's willingness to assist the Time Lord. He was also an expert chess player, able to beat the Doctor in 6 moves, though this may be because he was programmed with all chess games since 1886. K9 also had an aggression mode, revealed in State of Decay (Classic Series).

K9's mechanics included telescopic 'eye' probe, extendable nose that also concealed a powerful laser weapon, rotating ear-probes, flashing lights on the top and 'eye' panel, ticker-tape tongue and waggling tail antennae. K9's head was capable of moving up and down in nodding motions. Around his neck hung a tartan collar with a silver disk attached and his shell was painted in gold-grey metallic colour. Following K9 Mark I's departure, K9 Mark II had a slightly changed paint colour, though his most obvious change was his weapon, which was downgraded from killing to stunning only, no doubt a change included by the Doctor who built K9 Mark II. As well as this, K9 Mark II's memory and knowledge banks increased; a seeming mixture of K9 Mark I's, the Doctors and the TARDIS' knowledge and memories. After K9 Mark II's departure, K9 Mark III included front and rear wheel drive and a new coat of paint in metallic-blue, as well as a handle on top so he could be carried easier. Following K9 Mark III's demise in School Reunion (Series 2 : Episode 3), the Doctor built K9 Mark IV for Sarah Jane as a replacement. K9 Mark IV had a shiny metallic-blue paint job along with a number of improvements, including an omniflexible hyperlink facility and teleport functions. K9 Mark IV also had a laser weapon similar to K9 Mark I, though he did revert to his old, dark green colour at some point.

K9 Mark III being repaired by the Tenth Doctor in School Reunion

K9 Mark I
K9 Mark I accompanied the Doctor and Leela for many adventures, before Leela left the TARDIS to remain on Gallifrey in The Invasion of Time (Classic Series). K9 Mark I chose to remain with his "mistress" Leela on Gallifrey.

K9 Mark II
Prior to K9 Mark I's departure, the Doctor either obtained or constructed one or more copies of K9, which he used after previous incarnations of K9 left him. K9 Mark II was unpacked after K9 Mark I remained on Gallifrey, and travelled with the Doctor, assisting him and Romana in their quest for the Key to Time. The swampy terrain of Delta III however made it impossible for him to accompany them at all times. Towards the conclusion of the quest, the Shadow turned K9 to his evil will, during which time K9 Mark II assumed a more sinister personality in The Armageddon Factor (Classic Series). During K9 Mark II's travels, he contracted laryngitis and lost his voice, resulting in him remaining in the TARDIS for some adventures before it returned. Along with the Doctor and Romana II, K9 Mark II travelled into E-Space, where his head was knocked off while he tried to defend Romana, Adric, Varsh, Keera and Tylos. Unfortunately, K9 Mark II was damaged by the time winds and was unable to cross back into N-Space with the Doctor. K9 Mark II chose to remain behind with Romana and help her free the remaining Tharils from slavery in Warrior's Gate (Classic Series).

K9 Mark III
K9 Mark III was given to Sarah Jane Smith in 1978, though she did not receive the gift until shortly before Christmas of 1981, to serve as her companion. Sarah Jane and K9 Mark III travelled together on Earth before K9 Mark III broke down beyond Sarah Jane's repair, and she was unable to get anyone else to repair him due to his alien technology. The broken down K9 Mark III continued to travel with Sarah Jane in the boot of her car, until the Tenth Doctor met Sarah Jane and repaired him. After K9 Mark III's repair, he assisted the Tenth Doctor and Sarah Jane in defeating the Krillitanes, sacrificing himself to save them. K9 Mark III was destroyed in the explosion at Deffry Vale High School.

K9 Mark IV
K9 Mark IV was built or was already available to the Tenth Doctor, who left him behind as a replacement for Sarah Jane following K9 Mark III's sacrifice. K9 Mark IV also had a hidden compartment which contained Sonic Lipstick and a watch for detecting alien life for Sarah Jane.