My main character uses a fake name (let's say "Strider"). He introduces himself as Strider, and that's how most people refer to him.
Halfway through the book, he admits that his real name is something else (let's say "Aragorn"). He starts going by his real name after that.
When I'm writing my query and synopsis, and talking to my agent... how do I refer to him?
I mean, his name is Aragorn. But I don't want to write a query about "Aragorn" and then confuse people by handing them a book about "Strider."
In the query you call him the name that he uses in the first part of the book.
Lots of things change as the book unfolds; in this case your character's name is one of them.
The synopsis is a different kettle o'fish (Pail o'perch; bucket o'bass)
There you start out calling him Strider but when his name changes it's Aragorn (nee Strider.)
Nee is that clever, popular crossword clue that means "former name."
You used to see it a lot more when lady spouses almost always changed their names to that of gentlemen spouses.
(For example: Betty Buttonweezer nee Zippersneezer)