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

Grey's Anatomy has found Derek Shepherd's fourth sister.

The long-running ABC medical drama enlisted actress Amy Acker, of The Gifted and Buffy The Vampire Slayer fame, to play the role of Derek (Patrick Dempsey) and Amelia's (Caterina Scorsone) sister Kathleen.

The Gifted star will make her Grey's debut later this season, during the Amelia-centric standalone episode. She is currently committed to one episode of the series.

Acker's casting will see the last missing link of the Shepherd family in the Grey's Anatomy universe. Kathleen, who Meredith (Ellen Pompeo) has referred to as Kate in the past, has been mentioned several times throughout the show's run but has never been seen.

Kathleen is also a doctor like the rest of her siblings and specializes in psychiatry. Not much is known of Kathleen, though back in Season 9, Meredith gave her credit for being a good mother, revealing that she is married.

In another episode, as Entertainment Tonight writes, Meredith asked Derek's sisters to be contacted when it was revealed he needed a nerve donation following the injuries he sustained during the plane crash.

The series had previously introduced Neve Campbell and Embeth Davidtz as sisters Liz and Nancy Shepherd, in season nine and three, respectively. Amelia was first introduced on Grey's Anatomy spinoff series, Private Practice, where she remained a series regular for the last three seasons.

A few years later, Scorsone began to recur on Grey's Anatomy and was promoted to series regular in Season 11.

The series announced the Amelia-centric episode in January, following the news ABC had ordered three more episodes for Season 15.

"I knew for a long time that we were making 24," Grey's showrunner Krista Vernoff told press when the news first broke. "We've been making 24 episodes a year for many years now. ABC asked about [doing one more] and I spent a week thinking about whether or not we could accomplish it. I [ultimately] said yes because I like to be a team player."

The extra hour allowed for a story Vernoff had been toying with highlighting Amelia to happen in this season. The episode is set to be written by Julie Wong and directed by Bill D'Elia.

Not much is known when it comes to the plot of the upcoming hour, other than the fact it will take place entirely outside of Grey Sloan Memorial hospital.

"It's not just, 'Let's follow Amelia to the neuro lab," Scorsone told press. "It's definitely a little standalone. It's almost like a little movie on its own."

Grey's Anatomy airs Thursdays at 8 p.m. ET on ABC.