4. Bill Potts
As the first new companion for three series, the introduction of Bill Potts at the beginning of Series Ten was cause for excitement in the Doctor Who fandom.
Working undercover as a University Professor while guarding a safe in which Missy is imprisoned, The Twelfth Doctor takes an interest in Bill Potts, a canteen worker who attends all his lectures. Seeing Bill's unrealised potential, The Doctor invites her to officially join his class, and becomes her tutor. However, Bill eventually becomes caught up in The Doctor's other dealings when her love interest, Heather, disappears into a mysterious puddle and becomes a strange living fluid creature, which then pursues Bill, forcing Bill to flee with The Doctor through time and space. While The Doctor initially plans to erase Bill's memory of these events, she talks him out of it, and instead becomes his new travelling companion.
Bill's debut episode, 'The Pilot', is considered a great jumping on point for new potential Whovians. Despite only appearing for a single series, many fans believe Bill was a better match for the Twelfth Doctor than Clara, whom he inherited from the Eleventh Doctor.