Castor is the teacher of Henchman Training at The School for Evil, and Assistant Welcoming Leader. He is one half of a twin-headed dog with his brother, Pollux.
In the book, Castor is described as being "rabid, drooling, and male, with a grizzly mane." In the official ever-never handbook, he is black with red eyes.
Castor is rude and intimidating, frequently yelling and threatening anyone he can, especially his brother, Pollux, from whom he frequently takes their shared body, leaving Pollux to find makeshift bodies. However, Caster seems to be capable of a softer, calmer side as shown when he commends Sophie for causing a golden goose to give up her power and when he is talking to the staff about removing Sophie from the Trial by Tale.
"I'M CASTOR, WELCOMING LEADER ASSISTANT AND EXECUTIVE EXECUTIONER OF PUNISHMENT FOR ANYONE WHO BREAKS RULES OR ACTS LIKE A DONKEY."