Craig Owens acted as the Doctor's companion in the absence of Amy and Rory. He's a once-off companion like Jackson Lake or Wilfred Mott.
Craig Owens, Jackson Lake, Wilfred Mott, and others are individuals who might have gone with the Doctor but circumstances prevented them from leaving their lives to travel with the Doctor. For Craig it was Sophie, for Jackson it was his son, and for Wilfred it was his Daughter and the rest of his family.
- To expand on the above, there is intense debate in fandom over who qualifies as a companion, with some only considering those who travelled with the Doctor for a period of time as qualifying (so Liz Shaw isn't a companion by that criteria), while others follow the BBC's lead in deciding who is a companion (therefore Wilfred Mott, Lady Christina, Craig Owens and the Brigadier are companions). And others are a bit more inclusive, bringing in characters such as Sara Kingdom, Lynda "With a Y" and Sally Sparrow.