This development has been hinted at since season 1, but the recent Daredevil: Born Again S2 finale saw the deeply troubled Dr. Heather Glenn don Muse's mask.
In the season 1 episode "Art for Art's Sake," the twisted serial killer known as Muse is revealed to be Bastian Cooper (Hunter Doohan), a patient of Glenn's. Cooper goes to see his therapist, and after painting her a clear picture (literally) of who he really is, ultimately takes her captive and begins to drain her blood before Daredevil intervenes.
After a brutal battle with the Man Without Fear, Muse turns his attention back to Heather, who winds up shooting her assailant dead.
Some fans felt that Muse was "wasted" here, while others weren't fully on board with this depiction of the character in the first place. Muse's death did leave some questions heading into season 2, as we had been told to expect a "multi-season arc" for the villain.
Glenn went down a pretty dark road this season, and was haunted by frequent visions of the deceased murderer. In this week's episode, she finally put the mask on, and set photos have confirmed that she will fully adopt the Muse persona in season 3.
In the comics, Muse was an Inhuman whose spirit possessed the second incarnation of the villain, Morgan Whittier. Born Again has somewhat shied away from any overtly supernatural or magical elements (White Tiger's amulet, for example), so we don't expect Glenn to be literally guided by her former patient's ghost in the show.
Actress Margarita Levieva has now shared a behind-the-scenes look at "Lady Muse" via Instagram.
Across eight gripping episodes, survival, resistance and redemption collide as the battle for the soul of New York begins," reads the season 2 synopsis. "In Season 2, Mayor Wilson Fisk crushes New York City underfoot as he hunts down public enemy number one, the Hell’s Kitchen vigilante known as Daredevil. But beneath the horned mask, Matt Murdock will try to fight back from the shadows to tear down the Kingpin’s corrupt empire and redeem his home. Resist. Rebel. Rebuild."