The Master of the Curtain Mist, whose real name is Popokura, is the Sage of Water. He is an old man wearing blue robes and a blue Wizard-type hat.
He is corrupted by the Diva of the Twilight and forced to battle Isaac. When Isaac wins, he teaches Isaac Duo Runes. After finding that Twisted Wand Castle is barred off, Isaac battles the Master of the Curtain Mist again, this time rewarding Isaac with the Lost Magic of Water.
The Master of the Curtain Mist, like the other Sages, does not give his real name. His real name can be discovered by selecting him in a multiplayer battle.