I think the most common word used to express shock, disappointment or anger is f**k.

You can say that you have a "rude awakening" when you are shocked to realize that a thing or person was not as it seemed: i.e.: Finding out that he was really an unpleasant person was a rude awakening.

Are you looking for idioms for the experience of shock or for describing a person who is not who she says she is? I can't think of any that captures the feeling of shock at finding out someone is not who you thought they were (although there may be some), but here are some idioms for shock: It threw me for a loop (more mild surprise) I was floored My jaw dropped At a loss for words taken aback For more idioms about a person, another one is "true colors." So, you could say, for example, "I was floored when he showed me his true colors."

I couldn't really think of an exact idiom that describe this, and I am sure others will post some. But in the meantime, I thought of the following : 1) "his true self comes out" (not really an idiom, but an often used description) 2) "he was a wolf in sheep's clothing" (not exact because it describes a person who is pretending to hide his true nature)

“Damp squib” is one of them. This is used in British English. I don’t really know the American English, though. It means that something/someone is disappointing/less impressive than expected. You could say: He is (such) a damp squib! He turned out to be (such) a damp squib! Hopefully my answer is not a damp squib, though! I hope that helps. :)