Brain Wiz first appears in the episode "Wizard Battle." He tries to leave the battle when the Grand Master Wizard says that the prize is not an "open mouth kiss" (as he seemed to want). The Grand Master Wizard turns him into a cat, saying, "No one is allowed to exit Wizard Battle!"
His head looks like a brain. He wears a dark purple mask and seems to have yellow eyes. He wears a white suit that has dark and light purple designs on the chest and collar, along with a purple cape and dark purple gloves. His arms are about twice as thick as most humanoids' arms until about where the elbow would be, where they are normal width. In cat form, his fur is the same color as his head. In the episode "Reign of Gunters," after Ice King enters the wizard market, Brain Wiz can be seen at the background when Ice King landed on the ground. It is shown that he is still in his cat form making it possible that the Grand Master Wizard's magic is still affecting Brain Wiz unlike Finn, who rips out of his body when he is turned into a cat after forfeiting from Wizard Battle.
He was shown briefly in his regular form in "Wizards Only, Fools," implying that the cat spell had worn off, or he had overcome it.