The upcoming Gossip Girl spin-off has completed its first piece of casting.

Veronica Mars and Gilmore Girls alumna Krysten Ritter has been tapped to play Carol, the troubled older sister of Lily van der Woodsen (Lily Rhodes) in the new show, which chronicles the teenage years of Kelly Rutherford's alter ego.

Described as a sweet girl but also a bit of a disaster, Carol is an actress who's constantly making bad decisions in both her work and personal lives.

Lily ends up moving in with Carol after a falling out with mom and dad.

Krysten Ritter of Veronica Mars, Gilmore Girls and now Gossip Girl spinoff fame.

Circa '80s Lily and Carol will both be introduced in the May 11 episode of Gossip Girl, presumably through Lily's flashbacks in some form.

Meanwhile, casting for the lead role of Lily continues - and EW's Michael Ausiello says that if Gossip Girl producers Stephanie Savage and Josh Schwartz end up getting their first choice, there will be a lot of very happy fans out there.

Who could it be!?  

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.