What theories do people have against Jesus NOT being a real person? Not about whether he did miracles, or was the son of God, or etc., but that he is 100% a fictional character?

Because majority of historians generally agree that he at least existed. So I'm curious what would make someone say, "Nah, he's a fake guy".

Am I misunderstanding these statements as, "He was real but everything about him was made up, so he might as well be a fake guy"?