At first, little was known about Dr. Mark Sloan, the new McSteamy doctor on Grey's Anatomy, but that doesn't mean he wasn't noteworthy.
In fact, his being shrouded in mystery for much of the series only increases his appeal. That, and he's McSteamy. He's the man, revealed in Season Two, who was the catalyst in Derek and Addison's divorce. It was upon walking in on Mark and Addison in bed together that Derek fled New York for Seattle.
There, the neurosurgeon began a romance with an intern named Meredith, and the rest is history. Well, sort of.
When Addison eventually came out to Seattle, Derek took her back and they tried to reconcile. So when Dr. Mark Sloan appeared one day, he was not well received... by Derek's fist. He sutured his own face after being cut.
Mark's purpose was to take Addison back to New York, as he is very much in love with her. A plastic surgeon, he ended up assisting on a rare operation during his brief stay and giving career advice to Alex.
Despite his best efforts, though, Addison did not follow Mark back to New York. But in the second episode of the third season, he appeared wearing nothing but a towel in Addison's hotel room, after receiving a transcontinental booty call.
Mrs. Dr. Shepherd had just learned of her husband's affair with Meredith and embarked on a serious bender - one that ended with Mark back in the Emerald City. It was perhaps the most talked about moment of the season, and despite the fact that Addison didn't want to date him seriously, Sloan stayed in Seattle.
As a full-time plastic surgeon on Seattle Grace's staff, Mark tried to woo Addison, slept with Callie, hit on Meredith, terrorized Alex, and did all the other things that insecure, self-destructive Mark does. He even made a bid for Chief of Surgery. Still, relatively little is known about him and we look forward to seeing this hot character evolve.
Mark: Don't make me choose between you are her.
Lexie: Why? Because you'll choose her?
Mark: Yeah. I'll choose her.
Lexie: I think our relationship just ended.
MEREDITH: "McSteamy! Woo hoo!"
MARK: "McSteamy? That's what you're calling me now?"
MEREDITH: "Yes... but I don't think you are supposed to know that."
MARK: "How's my favorite dirty mistress?"
MEREDITH: "No, now I'm an adulterous whore!"