Timmy becomes Rex Perfection using Mark Chang's fake-i-fier. Rex is a Prince that can heal things with the shine from his smile, and is irresistible to everyone including Trixie Tang, whom Timmy was trying to impress with the costume.
Timmy Turner needed a costume for Trixie Tang's costume party, but because it was a competition his fairies could not help him since Da Rules forbid cheating in competitions using Fairy Magic, so Timmy used Yugopotamian technology instead: Mark Chang's fake-i-fier device. The device turned Timmy into Rex Perfection, and he was easily able to impress everyone at the party including Trixie. However, someone bumped into him, and the fake-i-fier when into shuffle mode, causing Rex to transform into a number of monsters which caused Trixie to run away from him. Eventually, Timmy's identity was discovered by Trixie, who threw him out of the party.