K9 first appeared in 1977 and is most commonly voiced by John Leeson.  Although not originally intended to be a recurring character in the series, K9 was kept in the show following his first appearance because he was expected to be popular with younger audiences.  he went on to have a starring role in The Sarah Jane Adventures and the pilot episode K9 and Company and also a later non Beeb production of K9 in 2009.