Alex O'Loughlin and Larisa Oleynik

Talk about pressure. Larisa Oleynik debuts on CBS' Hawaii Five-0 on March 21 as CIA analyst Jenna "K" Kaye, then returns for the season's last two episodes — giving her three shows to prove herself worthy of being signed as a permanent team member.

"They'll let me know by June 30 if I'll be a series regular in the second season," says Larisa, a former kid star on Nickelodeon's The Secret World of Alex Mack. "One of the reasons for bringing me in is the four [main actors] are being worked nonstop. And maybe to add more female energy."

Jenna arrives in Oahu from Virginia, on the hunt for Wo Fat, who killed her fiancé (also a CIA agent). "She's left everything she knows and has nothing to lose," says Larisa. "She thinks she's going at it alone — until she meets McGarrett."

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