Clara Zelverhaus used a sugar mouse as one of the enemies for her nutcracker to fight. She imagined him to be a monstrous, seven-headed rat, who the nutcracker defeated. However, he began to desire—first to be eaten, but then to eat, and that wanting allowed him to become real. The Rat King cut off six of his heads, escaped from the cabinet, and took up residence with other rats in the walls of the house.
During Nachtspel, the Rat King approached the nutcracker to talk. He told the nutcracker his journey to becoming a real rat and made the nutcracker confront the truth: he was not real unless he was with Clara. The Rat King advised the nutcracker to strengthen his desire and become real, and warned him about Droessen.
When Droessen tried to throw the nutcracker into the fire, the Rat King led his rats to attack the clocksmith, freeing the nutcracker. He reminded the nutcracker to want, thereby helping him become real.
The Rat King had grey fur, pink claws, a pink tail, and a high and reedy voice. He wore a felt cloak and a crown atop his head.