Gossip Girl Exclusive: Meet Blair's New Nemesis

A new pot-stirrer is headed to the Upper East Side, and she’s threatening to derail Blair Waldorf’s big fat royal wedding.

Sources confirm to TVLine exclusively that French actress/model Roxane Mesquida (Kaboom) is joining the cast of Gossip Girl as the meddling sister of Blair’s fiancée, Prince Louis.

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Little else is known about Mesquida’s princess character, save for that fact that she is not happy about her brother’s impending union and is hell-bent on sabotaging it. Oh, and the role is recurring, so she’ll be around long enough to do some damage.

Gossip Girl‘s fifth season — its first without Taylor Momsen and Jessica Szohr — is slated to launch on Monday, Sept. 26 at 8/7c.