Who's Blair Waldorf's new mystery man on 'Gossip Girl?'

Spotted spending time with Blair Waldorf (Leighton Meester) and also spotted in a sweaty squash match with Chuck Bass (Ed Westwick): some guy named Marcus.

Who's the new dude on "Gossip Girl?"

His real name is Patrick Heusinger. He's listed on IMDb.com and has appeared in "Skin-Deep," "The Nanny Diaries," "Tie a Yellow Ribbon," and "Sweet Land."

And he was in the 2006 US touring company of "Spamalot." So the dude can sing and dance. Makes sense since he's also a graduate of Julliard.

"He's Blair's new love interest and Chuck's rival," the source told People. "Blair met him in Europe, introduced by Ben, whom she flew to Italy with after Chuck stood her up."

So what's he doing playing squash with Chuck?

Observers on the set at Eastern Athletic Club in Brooklyn Heights told People that Chuck may be pulling a Georgina Sparks (Michelle Trachtenberg) by palling around with the newcomer, earning his trust and then trying to screw it all up for Miss Bee.

At the end of the scene, observers watched Marcus and Chuck shake hands.Then Marcus gave Chuck some digits that he entered in his cell. Uh-oh.

Photos: WireImage and Julliard.