We knew that Matt Lucas' Nardole, who first showed up in "The Husbands Of River Song", was going to be in more than one of the new Doctor Who episodes. But now we have a few more details about what he's doing and how long he's staying.
After a screening of the Christmas special, Lucas said, "You're stuck with me for a little bit," and said his character was more of a "valet" than the traditional companion. He added that, over the course of his episodes, "You learn a little bit more about me and I'm a bit of a thorn in [the Doctor's] side. I have a job to do, but I think [the Doctor will] regret giving me that job."
We still don't know exactly how long Lucas is around for, although it seems like it will be for at least a few episodes at the start of the season. Lucas said the dynamic between Nardole, the Doctor (Peter Capaldi) and new companion Bill Potts (Pearl Mackie) is "novel" because she's the only human among two different aliens. "She grounds us. There's something very contemporary about that character I think, and her performance," Lucas said.
It is true that the Doctor has had many fewer nonhuman companions than human ones, and it's been a long time since the TARDIS housed one. We'll have to wait until April to see how it all shakes out.