Dr. Diaper, or Dr. Nappy in the UK versions of the Captain Underpants books is the primary antagonist in The Adventures of Captain Underpants, and the very first nemesis of Captain Underpants. His plan was to blow up the moon with the Lasermatic 2000 and all the big hunks of it would come crashing down on earth, crushing every major city. This chaos would allow him to take over the planet.
Dr. Nappy appears in All Star Smashers as a Support Spirit, as a Novice class, wher you'll have to fight Go!Caillou and multiple Action 52's at the Weewoo Warehouse to obtain him. When fighting them, you will only have to defeat Go!Caillou to win. Also, he will start the match off wielding a Magic Bottle, and will be easily distracted by Items.