Why Every Modern Companion Left Doctor Who
2. Millie Gibson - Ruby Sunday
Whatever the truth of the scurrilous tabloid rumours that Millie Gibson was "dropped" from Doctor Who, we know that she left at the end of Empire of Death, as the Doctor left Ruby to spend time getting to know her birth mother. Russell T Davies was keen to point out that Ruby was not leaving, and that her TARDIS departure was part of the overall story being told.
RTD told SFX magazine that Millie Gibson was "Not leaving. Not at all. We were ordered for two years of a series off Disney, and we're delivering two years, and the Ruby Sunday story literally spans those two years,"
Indeed, Ruby played a very important role in season 2, leading her own episode and introducing the audience to the villainous Conrad Clark. Ruby's story ended with her convincing the Doctor to go and save Poppy from being written out of existence by reuniting her with her mother, Belinda.
It was a pretty neat resolution that appears to confirm that Millie's reduced role in season 2 was always the plan.