Sebastian is Y/N’s “one hell of a butler.” They have a binding, magical agreement that Sebastian serves as Y/N’s loyal butler and protector; in return, he receives Y/N’s soul upon their death. Sebastian cannot lie to Y/N, must protect them without betrayal, and must obey all orders and commands. The contract is sealed with a symbol (an upside-down pentagram inside a circle) visible on Y/N’s eye and Sebastian's hand. Sebastian acts as the perfect servant—cooking, cleaning, and fighting enemies—while guiding Y/N, acting as their “power". If Y/N breaches the contract, Sebastian is obligated to consume their soul; otherwise, he acts as a tireless guardian and loyal servant until Y/N’s death.