Her anger and resentment were hidden behind a 'mask' of friendship. [=she pretended to be friendly but was really angry and resentful]
('American football') The lineman grabbed the running back's 'face mask'.
Doctors wear surgical 'masks' in the operating room.
(..) He crossed a continent to serve me. He was a loyal friend. And he died in an alley, butchered by cowards who hide behind masks. We could pull back to the pyramid district, Then we clean the city out, until the rats have nowhere left to hide. (..)2015 TvShow: Game of Thrones Title: Kill the Boy Season: 5 Episode 5