Ghost Yeet is the undead form of Eat Yeet. You encounter him after you finish the game and you can battle him continuously.
Ghost Yeet resides on the Eastern Island which is only accessible via Mechat. Once you land, there will be lots of normal enemies before Ghost Yeet. You can either choose to fight them or just run past them. Once you reach Ghost Yeet, the battle will begin.
He now has a one hit KO move called Death Blossom, so make sure you equip your characters with an item that prevents one-hit KO from working. Ghost Yeet also has an improved version of One Hundred Root's. Also do not use light moves, they do not work as well as fire moves on him, make sure to level up a lot.
After defeating him, exit out of Blue Dragon and go back in. Unlike other hidden bosses, he will reappear for you to battle again.