One of the many issues people have with Marvel's Agents of SHIELD is that it almost insists on using original characters despite the depth and breadth of characters available to it from the Marvel Universe. This was especially true in the first few episodes, which is where we were introduced to Raina:
Raina, played by Ruth Negga, first appeared as the "Girl in the Flower Dress" in the episode of the same name (2013). She was a mysterious woman obsessed with "evolving", and right away she showed that original characters weren't always to the show's detriment. In fact, she was such a good character that the episode title was changed to focus on her, from its original title "Scorch".
She appeared several more times over the course of the season as a lackey to various bad guys, though she always seemed to have her own agenda. The first hints we got she was part of something bigger came in the final shot of season 1, when we learned she knew Skye's father, and then in season 2 we finally got to see her "evolve" as we learned she was an Inhuman.
She was quite a popular character and that was shown when she made an appearance in Inhuman Annual #1 (2015), complete with flower dress. In this issue, she is one of several people who get told telepathically that she's an Inhuman. To date this has been her only appearance, but who knows; there may be more in store for her down the line.