He did not use contractions such as "you've" for "you have" and addressed others by titles such as "Master", "Mistress", "Doctor-Master" (to refer to the Doctor) or "Young Master" (Adric, Starkey and in one instance, Clyde). Though he did not seem to resent his subordinate status, he sided with the Doctor's companions over the Doctor and showed a dark side, regarding other artificial intelligences with contempt. (TV: The Invasion of Time, TV: The Wedding of Sarah Jane Smith, TV: Robot Gladiators) He relished a brief chance to act as a figure of power. (PROSE: The Well-Mannered War)
K9 returned when Ship used the black hole as fuel. Though Sarah Jane, Luke, Rani and Clyde were happy to have K9 back, Mr Smith was not. (TV: The Mad Woman in the Attic) K9 spent a lot of his time in Sarah Jane's attic at 13 Bannerman Road, Ealing, but left to defeat Travast Polong. He attended Sarah Jane's abortive wedding to Peter Dalton, where he helped the Tenth Doctor, Luke, Rani and Clyde defeat the Trickster. (TV: The Wedding of Sarah Jane Smith) After helping defeat the Slitheen family, Clyde brought K9 to Park Vale Comprehensive School to help him with his test. He helped defeat the Rakweed before returning home. (TV: The Gift) K9 went to the University of Oxford with Luke, but restored Mr White's original programming through a mobile phone.(TV: Goodbye, Sarah Jane Smith)
On 24 April 2006, The Independent, the Daily Star and The Times confirmed, following previous rumours, that K9 would be featured in a 26-part children's series, K9, to be written by Bob Baker. The article in The Times also featured a picture of the redesigned K9 for the animated series. The series will be a blend of live-action and a CGI K9.
Although the first incarnation of K9 does not appear again in televised Doctor Who media, he is the star of the 2009 K9 television series, in which he undergoes a sort of "regeneration" process from which a new, more sophisticated and futuristic K9 emerges; in the first series of K9, the character is transported to London c. 2050 by Professor Gryffen (Robert Moloney). Though on regeneration the character loses his memory of his adventures with the Doctor, he assists Gryffen and several teenage companions against a dystopian regime of "The Department". The upgraded K9 has new specifications, sporting a sophisticated head-up display, the ability to fly, and more powerful laser weapons.