K-9, voiced by John Leeson, was a robot dog and a companion of the Fourth Doctor.
K9 was the creation of Professor Marius, who had made the loyal robot dog to circumvent weight restrictions when travelling to the Bi-Al asteroid. When he returned to Earth, Marius gave K9 to the Doctor. Capable of independent thought, and equipped with a blaster, K9 was a useful companion.
The Doctor gave K9 to Leela when he left her on Gallifrey, but had already prepared K9 Mk II. Despite numerous technical problems which put him out of action, K9 Mk II travelled with the Doctor until he was given to Romana as a parting gift. The Doctor sent another K9 to Sarah Jane Smith, after she had returned to Earth.