The character was teased by Patton Oswalt in the brief Season 2 teaser and now, courtesy of The Hollywood Reporter, we have confirmation that Bobbi Morse, aka Mockingbird, will in fact join the show, though maybe not as you'd expect.
Bobbi Morse has long been a fan favorite and one that many fans have expected to show up in the MCU at some point, heck she even had a show of her own in development at one point. Finally, the fanbase will get its wish with Adrianne Palicki joining the cast as Morse.
THR tells us that Palicki will join the show this season in Episode 5, with future appearances certainly a possibility, and it might not be as an ally to Coulson and his team at first. I'd suggest that someone of Palicki's stature is not going to be a one-off and that she is certainly being groomed for a little larger role than just guest starring on Agents of S.H.I.E.L.D. I can imagine her crossing over freely into Avengers: Age of Ultron, which is undergoing "reshoots" now or appearing in Ant-Man or Cap 3. Mockingbird is not an A-list Avenger, but she is certainly worthy of more than a cameo.
UPDATE: Here's the official release from Marvel.com. It doesn't add much but it does reiterate the point that Mockingbird's allegiances will be unclear at first.