If you think the romance between Dr. Derek Shepherd and intern Meredith Grey is complicated now, just wait until the love triangle begins! A woman from his past arrives in the eighth episode of Grey's Anatomy (Sunday at 10 pm/ET on ABC).

"I cannot say if I have a kissing scene with Patrick [Dempsey, who plays Shepherd], but I can say that he is a dreamboat and a cutie-patootie," teases Kate Walsh, the actress playing Dr. Addison Montgomery.

This hot lady doc is a world-class neonatal specialist who's called in to perform delicate surgery on a pregnant woman carrying conjoined twins. Turns out, the infants need to be separated while still in the womb. "There are only a few doctors who can do this in the world," she says, "and [Montgomery's] one of them."

As if she weren't busy enough with the OR drama, Montgomery will quickly agitate the Seattle Grace Hospital staff. "She arrives like a bat out of hell. What an entrance!" Walsh enthuses. "There is definitely tension, first with Meredith, then with Shepherd. Let's just say this kitty likes to scratch. She is just a girl you love to hate.

"And no, she is not [Shepard's] ex-wife," Walsh continues. "She is a character who brings [his past] to light, and it is very complicated. This is a big, big surprise." Walsh, who costars in the Will Ferrell film Kicking & Screaming (opening May 13), will stick around for five episodes.