This casting news comes courtesy of TV Guide, who posted the following (exclusive) report:
Acker will appear in at least one episode of the ABC series as Audrey, the famous cellist first mentioned in Marvel's The Avengers as being the love of Phil Coulson's (Clark Gregg) life. A talented musician turning heads in Portland, Audrey believes Coulson to be dead and has no idea that he's guarding her from a distance as a super-powered threat from her past re-emerges.
Acker's casting marks a reunion between the actress and Gregg, who shared the screen in S.H.I.E.L.D. executive producer Joss Whedon's Much Ado About Nothing adaptation. Currently starring on Person of Interest, Acker is also well known for her roles on Whedon's Angel and Dollhouse, where she worked with S.H.I.E.L.D. bosses Jed Whedon and Maurissa Tancharoen. Her credits also include Once Upon a Time, The Good Wife and Alias.
You might remember that the cellist to whom Coulson was romantically linked was previously referred to this season by Raina whilst tormenting the kidnapped Coulson (in the 11th episode), saying his love had cried for days after his "death". In episode 10, Coulson also described a woman to Ward who had been "particularly rewarding" to him, and spoke of her affectionately.
According to the report, she will first appear in the episode due to broadcast Tuesday, April 22.