Doctor Foster.

TV The Guardian recently published a brief but excellent piece from an academic about the implications of Doctor Who companion Bill's foster status:

"The first thing I noticed is that Bill is a working adult in her 20s, but still lives with her foster mother, Moira. Young people in care are often expected to become self-sufficient more quickly than their peers, but Bill’s situation is a nice example of the recent shift in policy that recommends young people have more gradual transitions to adulthood. Although we see Bill move out in episode four, this doesn’t work out, and by the sixth episode she is back living with Moira. I wonder how many viewers are aware that Bill’s experience isn’t the norm? How many would question the apparent ease with which Bill returned to live with her foster mother? In Scotland, less than 3% of young people eligible for support after leaving care remain with their former foster carers."

I suppose one of the disappointments of the series is that this back story element didn't resolve itself with any great unity. Although I suppose that's probably more realistic and akin to classic Who where companions would come and go and very rarely exited in a way which spoke directly about their character.