A Yellow Mustard is a large creature resembling a gold pot. Its lid is dark brown with a small, conical, faded orange spike on it. The base of the pot is also dark brown, with two short spikes on it. It has a short, gold, lightning-bolt shaped "wing" on either side of its body. Its large, red eyes are set in an angry expression. The Unversed emblem is on its back.
In Kingdom Hearts Birth by Sleep Final Mix, the Yellow Mustard receives a recolor that gives it a light yellow body which fades to orange towards its base. In addition, the brown sections on its lid and base become grey.
Much like the Yellow Opera heartless, The Yellow Mustard wields the element of Thunder in its attacks. It will either cast Thunder from a distance, or ram players with its electrified body. It can be swatted down with aerial attacks to stun it, or the player can attack from a distance with Shotlocks.