Grim, often referred to by the shortened name Grimo, is a central character who serves as the deuteragonist and familiar to the protagonist. His true name is Grimoire, and his origins lie in the demon realm as an entity of immense danger. Originally a demon of the highest class, he was considered a threat capable of leading the world to destruction. Unable to be killed by the forces of an entire kingdom, he was instead sealed within a forbidden book and kept in the royal family's underground library.

Grim's existence changes forever when he is discovered by the seventh prince. Initially, he attempts to manipulate and deceive his young master, scheming to take over his body and regain his freedom. However, when he peers into the depths of the prince's soul and witnesses the overwhelming and terrifying darkness within, his plans shatter. Overcome with a primal fear for his life, Grim abandons any thought of rebellion and submits completely, becoming the prince's familiar. This surrender is not born of loyalty but of absolute self-preservation, as he witnesses other powerful demons be destroyed in despair.

As the story progresses, Grim undergoes a significant and ironic transformation. Despite his origins as a calamity, he increasingly finds himself acting as the voice of reason and common sense. He is frequently horrified by his master's reckless magical experiments and godlike power, breaking into a cold sweat as he witnesses spells that pierce the atmosphere or combine elements in inexplicable ways. While the reincarnated prince shows little emotional fluctuation outside of his obsession with magic, Grim is highly sensitive and expressive. He often cries openly, moved to tears by tales of camaraderie or the plight of others, creating a stark and comedic contrast between the demon's emotionality and his master's stoic madness.

His relationship with the prince is the primary driver of his character. While their bond began with fear and coercion, Grim develops into a genuine loyalist and trusted attendant. He is the only one close to the prince who has been let in on the secret of his reincarnation, though he was too bewildered to fully process the confession at the time. Grim functions as a necessary brake, the only one willing to put a small check on his master's most dangerous impulses. This dynamic is amusingly one-sided; the immensely powerful demon is completely outclassed and often serves as a tool for his master's most outlandish spells, such as being absorbed into his hand to enable triple chants.

Within the story, Grim plays an essential support role. He is the prince's closest confidant and assistant, often present during magical research and combat. While his power is insignificant next to his master's, he is a formidable fighter in his own right. In his true form, he resembles a Baphomet, his power so great that he once blew away a Chimera Black Dragon with a single punch. However, he typically resides in a much smaller, cuter goat-like form to conserve magical energy and avoid causing discomfort. He can also assume a humanoid shape, appearing as a silver-haired young man with horns and a tail. He has grown even stronger by receiving magical power from other demons while serving the prince. Grim acts as a reliable combatant, a meter by which the strength of new enemies is measured, and a capable ally who can buy time in battle until his master arrives.