When your body has been taken over by that ghost guy and after you get..

Go back to creepy steeple and hit the health block, then save. Jump down into the well next to the hole you roll under in the wall.
When you are in the well run into the next room and kill all the enemies.
When they are all dead get out 'Vivian' from your party menu and push the gate, if you keep on pushing it it will move.
Push it to the top and get vivian to pull you into the ground and wait for the gate to go over you then pull yourself out and run on ahead. When you go into the room where you are looking at mario through a window go to your left and roll through the gap.
In the next room there are 2 chests and a parrot open up both chests and one of them should have the lower caps 'p' in it when you have that stand in front of the bird so you can talk to it but instead of talking to it get vivian to pull you under into the shadows and the bird will say out the freaks name.
P.s I will not tell you his name myself!

The guys name is Doopliss and you can't use the lower case p because you actually have to find it (to stop people who already know his name writing it in for a shortcut). To find the lower case p you have to go into the well in front of the castle where you first met him. Down there there is several enemies, after beating (or evading) them go to the right and you will see a grille on a slope. Push the grille about halfway up then use Vivian's special ability to hide in the ground so the grille slides over you. Go through the door. While following the tunnel you should see a box. Open it to release a lot of boos (200). After getting an ultra shroom from the boo that asks you how many you released go through the door at the far end for a room with two more boxes (another shroom and the letter p) and a bird. Stand in front of the bird and use Vivian's special ability to hide and he will moan to himself about how Doopliss treats him .Go back to where Doopliss confronts you, tell him his name (spelled exactly the way the parrot said it) then go back to where you fought him before he took over your body for a rematch.
P.S during the rematch only go after him because if you beat Goombella he just brings out Koops,then Flurrie etc

It's Doopliss and that lower case p it's notin all I did is grab it and said Doopliss to the prankster\ghost guy who stole you body your welcome

His name is Doopliss and you need to get the lower case p by going with Vivian down the well and beating the Buzzy Beetles. Make your way to the parrot and use Vivian's ability to hear the parrot say Doopliss. Then open the chests for badges and the letter p. Then run into Doopliss and tell him his name. He'll freak out and run to the Creepy Steeple. Follow him to Creepy Steeple and go to the top room again. Beat him and get your body, name, and Vivian at the same time.

Hope I Helped!
Mr Mario 4 U