Does it work if a character turns out to have another identity? For example, a seeming normal human turns out to be an angel or something.

I think that is a great idea. Rosewell the TV series has humans who are actually aliens. I miss that series... sniff...

it's worked for many authors... will work if you can write well and make it make sense to the reader... won't work if you can't...

You can find many examples in literature. Taking the Christian bible as literature, Jesus appears to be a man but is the son of God. Bram Stoker's Dracula appeared to be a man, but was a creature that practically defines horror to this day. J.R.R. Tolkein's Smith of Wooton Major appeared to be a man, but was one of the elven folk. Many characters in Stephen King novels appear to be normal humans, but are not.

Even in more "ordinary" stories, people assume a particular character has certain traits, but in a crisis they discover he or she has completely unexpected properties - heroic or foul.

As mammamaia points out, it depends more on how you present it, how believable it is.

I'm stealing your idea! :)

Oi u thief LOL!!!!!!!

Try this- make him seem more special than he is, a la the Wizard of Oz or Galaxy Quest- seems impressively skilled/powerful/unique but turns out to be normal. Hell, take it a step further, and make him appear impressively skilled, then he turns out not to be, and it later turns out he was just playing at not being skilled but he really was. I dare anyone to go a step further.

I think it will work. Give it a try.

Im leaving the identity open to interpretation. It could be an angel or even Hashem himself in human form.