The news is in, and the new companion has been chosen. Officially announced on April 23rd, 2016, Peter Capaldi's Doctor has a shiny new companion.
The twenty-eight year old actress hasn't been in the biz for very long. According to IMDb, her acting career began in 2013 when she starred in Svengali as Foh Girl. Which also starred Vicky McClure, Roger Evans, Martin Freeman, Maxine Peake, and Jonny Owen. Mackie was also in Doctors, as Anne-Marie Frasier.
It is no secret that picking a new companion is incredibly difficult. Mackie was apart of a list of seventy other potentials. Seventy! They (they being, Steven Moffat, Brian Minchin, Andy Pryor, and Time Lord himself, Peter Capaldi) had to whittle down that number, which they succeeded in doing so. They came down to five stars that could be the next person to be standing next to the Great Time Lord.
"Myself and my team looked loosely at about 70 people, met about 50, and then recalled about 10. Then we brought in the final five with Peter Capaldi."
~ Andy Pryor
Pearl Mackie plays Bill. As of right now, little is known about Bill, and her origin story.
Doctor Who is to have its annual Christmas episode, introducing Bill.