After acquiring the National Dex, go to Canalave City and talk to Sailor Eldrich. Answer "No" to his offer, then fly home (Twinleaf) and save and turn off the DS. When you restartthe game, it will start with a quiz (like the interviews). Choose a word and select "Confirm". Your character will look like the selected word. For example, using the word AIPOM will make your character be dressed in an "Aipom suit". The trainer card will now show the sprite in an Aipom suit. Recommended words are CHARIZARD, BATTLE, ARCEUS, and LINK.