Mark

Mark is the Doctor's companion! They met in a middle of a Night, in a Dalek war before getting teleported to Hielbentu. Mark is smart, a friend to the Doctor. They later met Hannah, a girl during an expermental night on Earth in 2016. They saved the girl from a machine and left Earth. Later on, the gang met the Master and then went ahead in time.

He left the Doctor in the episode End of the Line.