When your on your way to Twilit Town to get your 4th star you will have to fight a ghost that when you tattle on him it says he has no name in creepy steeply. While your fight him he will transform into you and when you beat him he takes your identity. Then on your to Twilit town he will appear and you have to guess his name and when you get it wrong he will fight you but neither of your or his attacks will work you will have to run away and go to town and go talk to Vivian. once she joins your team go to creepy steeply and down the wail once in go to the bared door and push it up then go into the shadows and go through th door and get the treasure chest get the P go to the impostor Mario and when you guess his name say Doopliss. Then go back to his castle and you will have to fight him and your partners.

P.S. Vivian will leave your party for a while but will come back

P.S.S Focus on Doopliss