Jiaying

Jiaying is the hidden true main antagonist in the second season of Marvel's Agents of S.H.I.E.L.D. She is the leader of the Inhumans and Skye's biological mother. Her abilities include regeneration and semi-immortality. Unfortunately in order to live she has to drain life force from other Inhumans.

She's portrayed by Dichen Lachman.

Overview
In the second half of season two, Skye met her for the first time after being transported to the Inhuman facility. She taught Skye to harness her powers and eventually took her to her father, Calvin Johnson. It turns out they were still married. In the season finale, SOS, she revealed her true colors: she was a ruthless, manipulative woman who hated S.H.I.E.L.D. and would do anything to protect the Inhumans, even if it meant destroying S.H.I.E.L.D. She led an attack on the SHIELD Helicarrier and killed a handful of the agents. She even tried to drain Skye's life force and stated that that was her true gift, proving that she cared for no one but herself. This ultimately led to her undoing, as she wound up being killed by her own husband, Calvin.

Trivia

 * Although season two had several villains, Jiaying turned out to be the true main villain of the season because she had bigger plans than anyone else, which made her the biggest threat of the season.
 * In the overall story of season 2, Jiaying is the main antagonist, Daniel Whitehall is secondary, Ward is tertiary, Sunil Bakshi is quaternary, and Dr. List is quinary.
 * She and her ex-husband Calvin Zabo were both called monsters, though in reality because of her manipulative and hateful nature, she was by far much more monstrous than Calvin.
 * Jiaying is similar to Magneto, as she wants her kind to live peacefully at the expense of humanity. They also have history with being persecuted, which is the reason for their evil plans.
 * Though she willingly does the things she does, her actions can be reasonably blamed on Whitehall as if not for the brutal torture he put her through, she never would have become the person she was in the aftermath of the trauma.