She guested in the first season, hunting escaped Asgardian convict Lorelei, and providing the first hints to the identity of the 'blue alien' behind GH.325 - who we now know is Kree.
This time out she will be found (armourless) by the SHEILD team with partial amnesia for, what producers describe as, 'some much-needed humor.'
Sif has no idea who Thor is or that she serves a king, but still has her childhood memories and knows that she's Asgardian. We've been dealing a lot with space aliens and Inhumans in our last few episodes - things our Earth-bound heroes just don't know much about," says executive producer Jeffrey Bell. "They need help understanding what's really going on. Sif's memory - what's left of it - will come in very handy."