Just in time for what could be the mid-season finale of "Doctor Who," we'll get introduced to a new companion. And this is a companion who really, really likes to talk.

Jenna-Louise Coleman has been cast as The Doctor's new companion for "Doctor Who," replacing Karen Gillan and Arthur Darvill, who are leaving the show by the upcoming season's fifth episode.

"It's not often The Doctor meets someone who can talk even faster than he does, but it's about to happen," showrunner Steven Moffat said in a statement to Entertainment Weekly. "Jenna is going to lead him on his merriest dance yet. And that's all you're getting for now."

And that's really all Moffat is offering. No name for the new companion has been announced, nor any of her backstory.

"Who she's playing, how The Doctor meets her, and even where he finds her, are all part of one of the biggest mysteries the Time Lord ever encounters," Moffat said. "Even by The Doctor's standards, this isn't your usual boy-meets-girl."

Coleman is the fifth full-time companion to join The Doctor since the series was revived in 2005. She follows a path that began with Billie Piper in 2005-06, continued with Freema Agyeman in 2007 and Catherine Tate in 2008, and culminated most recently with Gillan and Darvill, who have had the longest trip yet -- since 2010.

Coleman, 25, is still pretty new to television and movie screens. She premiered in the long-running British drama Emmerdale in 2005 as Jasmine Thomas, appearing in nearly 170 episodes, before moving to "Waterloo Road" in 2009.

In 2011, she played Connie in "Captain America: The First Avenger," and is set to play Annie Desmond in the upcoming miniseries "Titanic" from "Downton Abbey's" Julian Bellowes.

One interesting note about Coleman's casting is that she will be the first full-time companion since Piper to not have previously appeared in "Doctor Who." Agyeman played Adeola in the July 2006 episode "Army of Ghosts," before taking on the role of Martha Jones in March 2007.

Tate actually first appeared in "Doctor Who" in July 2006 as "The Bride" in "Doomsday," teasing her first appearance as Donna Noble in "The Runaway Bride" later that year for Christmas. She would reprise the role of Donna in April 2008 in "Partners in Crime."

Gillan played the Soothsayer in the April 2008 episode "Fires of Pompeii" before taking on the role of Amy Pond two years later in "The Eleventh Hour."

Gillan has portrayed the only primary companion to The Doctor under Matt Smith. Smith himself is entering his third season in the title role, and is expected to be back for a fourth in 2013.

"Doctor Who" is expected to return this fall on BBC and BBC America.