In Doctor Who, rebooting the Doctor is easy — the companion is not

polygon.com:

There are few things on TV that feel as good as meeting a new Doctor and just knowing they’ll be terrific. Doctor Who’s 21st-century run has had plenty of ups and downs thus far, but casting the titular role has never been one of them. Ncuti Gatwa, the Fifteenth Doctor, is no different: immediately charming, charismatic, and odd. In his first full adventure, “The Church on Ruby Road,” Gatwa displays shades of Doctors who’ve come before while putting his own strong stamp on the role; simultaneously welcoming and enigmatic, he’s the kind of person you’d happily follow anywhere but is clearly not telling you everything.

And in “The Church on Ruby Road” we finally meet Fifteen’s companion for the forthcoming season: Ruby Sunday (Millie Gibson), an orphan who finds herself in a stretch of very bad luck. The ensuing episode is a perfectly nice holiday romp, the kind that will sell you on both this new Doctor and Ruby Sunday in no time at all, a feat so tricky you might forget how easy it is to mess up. Doctor Who is good at picking Doctors. The hard part is picking the companion.

Modern Doctor Who has put a lot of weight on the Doctor’s sidekick, often to the show’s detriment. The role is arguably more important for Doctor Who to get right than the lead — a good companion (or in some cases, companions) has to both be the audience surrogate and the grounding force for the Doctor. Before showrunner Russell T. Davies left Who for the first time in 2009, his then-final stretch of specials emphasized that bad things happen when the Doctor is alone, stewing in his centuries-old mind as he travels all of time and space and confronts the horrors therein.

Where Doctor Who has gotten into trouble in the past is when its writers