Which member of the Velvet Room is not a playable character in Persona 4 Arena Ultimax?

The answer is Theodore. Theodore is not a playable character in Persona 4 Arena Ultimax because he was not introduced as a playable character in the original Persona 4 Arena. He only serves as the assistant and attendant in the Velvet Room, providing guidance to the protagonist throughout the game. Therefore, he does not have a role as a playable character in the fighting game.