Amy Acker is the latest addition to the cast of ABC's Grey's Anatomy.

According to EW, the actress will play Derek Shepherd's (Patrick Dempsey) fourth sister, Kathleen on an upcoming episode.

If you watch Grey's Anatomy online, you know that Kathleen has been mentioned on several occasions throughout the series, but we've yet to see her in the flesh.

Given that Derek is dead, the only real link to the action is through Amelia (Caterina Scorsone) who works at Grey Sloan Memorial.

We do know that Kathleen is a psychiatrist and that she will first appear in an Amelia-centric hour, but there's no telling what actually brings her to Seattle.

Neve Campell previously played Liz Shepherd, while Embeth Davitz played Nancy Shepherd.

It's crazy that, after all these years, Grey's Anatomy is still fleshing out some of the characters who have been on the show for years.

The show remains a huge success for ABC, currently ranking as the number one scripted show on the network with an average audience of 6.9 million total viewers and a 1.7 rating in the demo.

When DVR ratings are factored in, the show swells double digits.

The Amelia-centric hour was confirmed shortly after ABC extended Grey's Anatomy Season 15 to 25 episodes.

"I knew for a long time that we were making 24," Grey's showrunner Krista Vernoff told press when the extra episodes were ordered.

"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."

Acker currently stars as Caitlin Strucker on Fox's The Gifted. The show is not performing well in the ratings and is likely to be canceled.

Could Acker become a more permanent fixture on Grey's Anatomy?

That would be fun!

What are your thoughts on this news?

Hit the comments below.

Grey's Anatomy airs Thursdays on ABC.