[SPOILERS for the Doctor Who 2014 Christmas Special ahead!]
The Doctor Who 2014 Christmas special, "Last Christmas" (read our review), ended with Peter Capaldi's Doctor and Clara Oswald (Jenna Coleman) taking off together, reuniting for more adventures across space and time after the season 8 finale, "Death in Heaven", left it unclear what the future would hold for Clara and The Doctor. Now, we can officially confirm that Coleman will be returning in the ninth series of the sci-fi/fantasy series' modern run.
Coleman's predecessor, Karen Gillan as Amelia Pond (the companion to Matt Smith's Doctor), hung around on the show for two and a half seasons. It appears that Clara will, in fact, be around as least as long, as Coleman has specially stated that she will be getting "a whole other series of stories with the Doctor" - meaning, she will be around for the entirety of season nine, presumably.
Clara became a much better-developed character throughout Doctor Who's eighth season (a.k.a. Capaldi's debut series), and Coleman - who's always done a good job of bringing Clara to life on the series - also said that she thinks it's "wonderful" that the character's story will continue to develop for a whole additional series. Here is her quote on the matter (via BBC News):
"There is so much more to do. I think they've [The Doctor and Clara] finally just reached a point where they really understand each other. The arrival of the 12th Doctor has just kind of dropped this whole bombshell and allowed the dynamic to totally change, so I think just when Clara was feeling more comfortable in the relationship, it has suddenly thrown something new up."
Doctor Who introduced Clara as something of a glorified plot device (a la "The Impossible Girl"), whose existence drove the over-arching mystery in the second half of series seven. However, in series eight, she became more of a flesh-out individual, in no small part through her time with Danny Pink (Samuel Anderson), short-lived it may have ultimately been. Now, it remains to be seen if there is, indeed, more interesting story material to be told around the Clara character and her relationship with the most well-traveled of the Time Lords.
Steven Moffat is confirmed to be writing the season nine premiere for Doctor Who, after having served as its head writer for the past four series now. Series eight (under Moffat's watch) was on the whole much better-received critically than series seven, so there is probably now a bit more support for him to go on telling The Doctor's tale than there was this time a year ago, when Smith left the show (even after what seems to be generally lukewarm reactions to the season eight finale).
Still, just as any companion is only able to run with The Doctor so long, it's possible that Moffat will begin handing off more responsibilities to his fellow Doctor Who writers and/or producers, especially since he'll be working on Sherlock season four over the forthcoming year. If nothing else, Doctor Who season nine will be including some fresh creative input, what with Torchwood alum Catherine Tregenna joining the writing crew (making her the first female writer Doctor Who's had under Moffat's watch).
Doctor Who season nine will air sometime in 2015.
Source: BBC News