Marisol Nichols

NCIS: Los Angeles' Special Agent G. Callen (Chris O'Donnell) is about to get a very special visitor from his past. Marisol Nichols, who took on Jack Bauer on 24 and a neighborhood full of ghouls in the ABC summer series The Gates, has signed on to play Callen's ex-partner, Tracy Keller.

"The role's booked as guest star/recurring," says Marisol, who debuts in the sixth new episode, titled "Stand-Off." Word is the two characters, who share a very complicated past, will lock lips. This comes as great news to Marisol, who first fell for O'Donnell in 1992's Scent of Woman.

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