He replaces SMB1 Bowser and throws shells instead of hammers (and, unlike the original NSMB Bowser Jr, also spits fire, but he learned that in NSMBWii so we all know he can spit fire)
There is a minor bug where he sometimes jumps up into the shell he just threw and hurts himself, but it would be much more difficult to wait untill he kills himself than actually defeating him with the shells he's throwing, so technically no glitching is possible.
Credit for the sprites goes fully to Shocker, they were originally made to replace Ludwig von Koopa, I only adjusted them to the Bowser graphics and added the bandana

LunaDLL code:
NPCMemSet,30,0xE2,5,0,0,w
Screenshot: