I think the Masked Sith is....

Qimir's twin.

The way the Sith walks and looks at Osha after gliding down behind her in Khofar is very similar to how Qimir looks at her when she is pretending to be Mae after Torbin is killed. But Qimir is stuck upside down on the tree. And it's not like there can only be one pair of twins in the Star wars universe.