Gossip Girl's Georgina.

So the story behind her: She was Serena's old friend from her partying days who came back to ruin her life...and succeeded. She pretended to be some girl named Sarah and befriended her boyfriend Dan to steal him away, pretending that she didn't know Serena. Meanwhile, she had this tape of and held it as blackmail, so Serena couldn't tell Dan the truth. Michelle Trachtenberg did an amazing job with this role.