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.
Castor makes his debut in Boys Ruin Everything he is tied to Pollux, Pollux is in charge of telling students the rules about everything they will do in their first year while Castor mostly interrupted him many times just saying things to scare students. He was able to scare nevers as well. He is shown to be aggressive many times, but when Sophie made a golden goose give up it's powers Castor was surprised and spoke in a more soft voice. Later on he gives Sophie a dead quail as a gift for winning the trail by tale. He is petrified by the school master but brought back later on.
Castor remains the same in this book, however he and all the male teachers were put in a sleeping spell, he was able to hear everything as he was sleeping after Hort did a man wolf scream we awakened and started to clean up the many messes the boys did.
Castor was loyal to Rafal in this book.
Castor is a very big dog, he uses his teeth for combat
- When he retires he wants to raise sheeps so he can eat them for meals.
"I'M CASTOR, WELCOMING LEADER ASSISTANT AND EXECUTIVE EXECUTIONER OF PUNISHMENT FOR ANYONE WHO BREAKS RULES OR ACTS LIKE A DONKEY."