K-9 is a mechanical "dog" who sometimes accompanies the Doctor and his Companions. There have been several models of K-9.
K9 has a very consistent personality, across the models that have had some contact with the Doctor. He has a polite, formal manner with just a hint of pedantry bordering on condescension. Though he often displays feelings such as sorrow and self-regard, he has often stated that he has not been programmed to have emotions. He does not use contractions such as "you've" for "you have" and addresses others by titles such as "Master", "Mistress", "Doctor-Master" (to refer to the Doctor).
When the Doctor created K9 Mark IV to replace K9 Mark III, he added a shiny metallic blue paint job and a number of improvements over Mark IV including an omniflexible hyperlink facility. Later, though, K9 reverted to his old, dark green color. K9 displayed a teleport facility and utilizes a laser weapon similar to the previous K9's.