A key character from the X-Men mythos will return in "X-Men: Apocalypse," as Rose Byrne has signed on to the film to reprise the role of CIA agent Moira MacTaggert.

Entertainment Weekly reports that MacTaggert, who had her mind wiped by Professor Charles Xavier at the end of "X-Men: First Class," will meet the X-Men again for the first time almost 20 years after the events of that film.

"Apocalypse" writer Simon Kinberg said they plan to dig into her relationship with Xavier. "It's certainly a rich relationship that we started to dig into in 'First Class,' and would like to mine more in this one," he said.

Created by Chris Claremont and Dave Cockrum, Moira MacTaggert first appeared "Uncanny X-Men #96" and is a geneticist who runs the Mutant Research Center on Muir Island, rather than a CIA agent. She also helped Professor X found his school and create Cerebro.

Kinberg also said the film will bring closure to the character arcs for Xavier, Magneto, Beast and Mystique, played respectively by James McAvoy, Michael Fassbender, Nicholas Hoult and Jennifer Lawrence.

"They're such young actors and young characters, so there's a lot more story to tell. But as a first phase of their lives, there is this feeling of closure by the end of the movie," Kinberg said.

And with the film set in the 1980s, Kinberg said they are "looking at mutants that would fit into the period," and Dazzler "could happen."

Kinberg also spoke to EW about the upcoming "Fantastic Four" film reboot, noting the franchise could potentially cross over with the X-Men films. "If we wanted to find a way to connect them to the X-Men, we could," Kinberg said. "There's a lot of precedent from the comics."

Byrne's return follows the news from last week that three new actors would portray Cyclops, Storm and Jean Grey in the film. Alexandra Shipp will play Storm, Sophie Turner will play Jean Grey and Tye Sheridan will play Cyclops.

"X-Men: Apocalypse" is directed by Bryan Singer and also stars Oscar Isaac as Apocalypse. The film is scheduled for release on May 27, 2016.