Gooper Blooper appears as the boss of World 3, marking his first appearance in the Paper Mario series. Gooper
Blooper used to be a music-loving Blooper until a Royal Sticker landed on him and turned him and all surrounding Bloopers into Poison Bloopers. He is the source of the poison that plagues the forest.
In battle, Gooper Blooper's attacks are determined according to the music. In the beginning, he will start off with a double slap using his front tentacles. After that, he will use his back tentacles for a considerably weaker double slap. Once the music changes again, Gooper Blooper can slap with all four of his large tentacles. When the music changes once more, Gooper Blooper will turn dark purple, signifying he is about to spray Mario with poison. If Mario is hit with this attack, he will be poisoned for four turns. As well, parts of the screen will be covered in goop. Oddly, when Gooper Blooper is poisoned by his own attack (if you use the sponge before he spits poison at you), the battle music changes into a remix of the Super Mario Bros. theme. This game is the first time that Gooper Blooper speaks with dialogue.
At the end of battle, depending on how fast it took you to beat this boss, Kersti will appear to tell you that the battle took you a lot of stickers to beat. This boss is hard but if you used the sponge sticker when he is about to spit poison at you, things are easier.