Devil in disguise noun phrase
Used to imply that someone seems to be good, useful, or benevolent but in reality they are not
Don't be fooled by his appearance. He is a devil in disguise.
Suddenly she is being helpful to me. I doubt that there is a devil in disguise.
John's just pretending to be nice to me. I know he is a devil in disguise.
The origin of this phrase is not clear.
Used to express something bad is happening again
Here we go again! Lisa has just asked to borrow my new dress.