Apparently you need to make a small character named Trevor. I made a large character named that, and it did not unlock. to modify the smaller character, you need to go down the left hall way (from the main room on the carrier) and follow it to the end. There you'll be in a place where on the left, there's two character circles (to stand in and press square) this will allow you to character customize. choose any character and call him Trevor (a reference to Iron Man 3... ) Capitalization might be required. I did it as listed (big T, littler revor)
After that back out of the character customizer to save him and the trophy popped.