Depending on how you interact with the characters within the game can change how they interact with you. I'm sure there are more characters than Amy that you can do this with, but I don't know which. Here is how you get Amy to admit Dexter is imaginary, be nice to her up until the point where you go to Nocturnus. You may have to go through some of the questions instead of the skip response, but she should ask to be alone with you for a moment. Yet again, go through the remarks correctly and she wil admit Dexter was made up to make you jealous. I don't know anything specific, but you may have to go through other remarks in the game to basically say that she made him up. Just be nice to her when ever, and I'm sure you will reach the secret of Dexter.