Jenna-Louise Coleman
http://www.bbcamerica.com/anglophenia/2012/03/new-companion-unveiled-for-bbc-americas-doctor-who/
Lead writer and Executive Producer, Steven Moffat, says: “It always seems impossible when you start casting these parts, but when we saw Matt and Jenna together we knew we had our girl. She’s funny and clever and exactly mad enough to step on board the TARDIS.
“It’s not often the Doctor meets someone who can talk even faster than he does, but it’s about to happen. Jenna is going to lead him his merriest dance yet. And that’s all you’re getting for now. 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. Even by the Doctor’s standards, this isn’t your usual boy meets girl.”