'Grey's Anatomy' Casts Amy Acker as Derek's Mysterious Fourth Sister

Jessica Napoli

After 15 seasons of the ABC medical drama, Grey's Anatomy, we're finally meeting one of the most mysterious characters — Derek Shepherd's (Patrick Dempsey) fourth sister.

The Gifted star Amy Acker has been cast as the missing Kathleen (aka Kate) Shepherd for an upcoming episode which centers on her TV sister Amelia Shepherd, played by Caterina Scorsone, according to The Hollywood Reporter. Details about the plot and air date are still under wraps.

'Grey's Anatomy' Season 15 Episode 12: It's Britney, Bitch (RECAP)

'Grey's Anatomy' Season 15 Episode 12: It's Britney, Bitch (RECAP)

It’s the most wonderful time of the year all over again! 'Girlfriend in a Coma' took us back to the holiday season.

McDreamy's MIA sister has been frequently mentioned but this is the first time she's appearing on-screen. His three other siblings have been played by Embeth Davidtz (Nancy), Neve Campbell (Lizzie) and the previously mentioned Scorsone (Amelia).

Scorsone actually became a series regular before exiting to appear in the Grey's spinoff Private Practice only to return after that show was canceled. This season she's involved in a love triangle with Owen (Kevin McKidd) and Teddy (Kim Raver).

CATERINA SCORSONE

Meanwhile, the timing of a Kathleen's appearance is interesting considering Meredith (Ellen Pompeo) has moved on in the dating world and is feeling things for two people: Link (Chris Carmack) and DeLuca (Giacomo Gianniotti).

Grey's Anatomy, Thursdays, 8/7c, ABC

AlertMe