Dooplis makes fun of your name
When you've beaten him at first he'll become you then when he asks "Guess my name" say your name and it'll probably be wrong (depending on your name) then he'll say "Who would name their kid that!?!?!'' (try this with your friends and make fun of their name too!!!!!!!)
He's the guy who makes you a purple Mario and makes himself look like you.
We have more cheats and tips for this game here, Paper Mario: The Thousand Year Door cheats
Comments for this hint