Blackus Shadow Prince, also known as Blackus, is a supporting character in the series. He is introduced as one of the last two loyal servants of the protagonist, Leonis Death Magnus, alongside Shary Shadow Assassin. Despite appearing as a large black dog, Blackus is actually a royal wolf and holds the title of prince of the Realm of Shadows.

Blackus has a long and deep history with Leonis, having been his comrade and best friend for a thousand years since the era of the Demon Lords. They fought alongside each other on many battlefields, sharing a relationship built on mutual trust and cooperation. In the past, as the war against the forces of light was ending, Blackus was the one who persuaded Leonis not to participate further, arguing that he was the last remaining Dark Lord. This counsel directly led to Leonis’s decision to seal himself away in the Great Mausoleum, a plan Blackus supported by also entering a long slumber within the shadows.

Awakened a millennium later by Leonis’s call, Blackus serves as his master's primary agent for gathering intelligence on the unfamiliar modern world. The two cooperate seamlessly in combat, combining their abilities effectively. His most notable abilities are tied to his nature as the Shadow Prince; he can freely enter and move through shadows, using them as a means to travel across great distances instantly.

Upon awakening, Blackus finds his princely dignity challenged by his new environment. While wandering near Excalibur Academy, he is mistaken for a stray dog by Sakuya Sieglinde, one of Leonis’s new allies. She promptly "rescues" him and gives him the temporary name Kurogane Fluffymaru, a name he privately dislikes. Despite this, he accepts the food she offers and tolerates the collar she puts on him, demonstrating a pragmatic side to his personality. This willingness to set aside his pride when necessary forges an interesting bond between the ancient wolf prince and the modern holy sword wielder.