3. The Doctor's Attachment To K-9
Most of the time our leading Time Lord is referred to as "the Doctor" by his legion of faithful companions but there's been one that knows him only as "Master" (and variations thereupon). The companion in question is K-9 who first appeared in The Invisible Enemy as the creation of Professor Marius. K-9 is a dog-like robot who was programmed to be loyal and logical to whomever it considered its master and there have been a total of four versions (or "Mark I, II, III, IV") of him to date, with the most common, the Mark II, serving as the Fourth Doctor's beloved companion. He had the ability to warn the Doctor of impending danger and was a valuable asset to the TARDIS team. In the revived series, K-9 was reintroduced in the Series 2 episode School Reunion alongside Elisabeth Sladen as Sarah Jane Smith and, after being blown up by the Krillitane, the Tenth Doctor left Sarah Jane K-9 Mark IV to continue serving his "Mistress". As K-9 has only been seen a few times in the revived series, many viewers could be confused as to why he was such an important part of the Doctor's life. At least two versions were created or upgraded by the Doctor and, while most of the Doctor's companions have been human, K-9 proved that a dog really is a man's best friend indeed.