It's a bit creepy; she's a kid he rescued from his nutter vampire ex-lover (who clearly will pop back on these scene if the show last long enough) who wanted them to all become one happy family. There's a hilarious bit where he stabs her (the ex-lover not the kid) and sets her on fire while thoroughly inappropriate music plays.

I give it 4 as CBS probably can just jam CSI reruns in there if they don't have anything else.