Leonis Death Magnus is the main protagonist of The Demon Sword Master of Excalibur Academy. Originally a heroic swordsman named Leonis Shealto, he was discovered as an orphan by the warrior Shardark, who recognized his potential and trained him in swordsmanship. He rose to become one of the famed Six Heroes, known as the Hero of the Holy Sword. However, he was betrayed and assassinated by the nobles of his own kingdom, orchestrated by Arakael Degradios. Near death, a mysterious girl resurrected him and transformed him into a Dark Lord. Embracing this new purpose, he became the most powerful of the Dark Lords, ruling over the nation of Necrozoa and commanding overwhelming magical might. When the remaining heroes moved to eliminate all Dark Lords, Leonis chose not to fight his former comrades. Instead, he sealed himself inside a crystal coffin and cast a resurrection spell, planning to reincarnate a thousand years later. In the present era, his seal is accidentally undone by a young girl named Riselia Ray Crystalia, and he awakens in the body of a ten-year-old human child. Mistaken for a refugee, he is taken in by Riselia and enrols at Excalibur Academy to gather information about the changed world. His true goal, stemming from a promise to the Goddess of Rebellion known as Roselia, is to find her vessel, rebuild a new army of Dark Lords, and once again conquer the world.

In terms of personality, Leonis is highly responsible and mature, always clear about his objectives. He does not hesitate to annihilate enemies or anyone who stands in his way, but he shows a kinder and protective side toward those he considers his subordinates. This contrast between his childlike appearance and his commanding, ancient demeanor often raises suspicion among his companions. He is pragmatic, calculating, and adapts quickly to his new environment, hiding his true identity behind the facade of an ordinary academy student. He has a fondness for coffee, claiming its dark color fits the theme of a demon army, and dislikes vegetables, which Riselia forces him to eat.

Leonis's role in the story is that of the reincarnated Demon King navigating a world that has largely forgotten the ancient war. He becomes a student at Excalibur Academy, joining the 18th Platoon alongside Riselia, Regina Mercedes, Sakuya Sieglinde, and Elfiné Phillet. His key relationships include Riselia, whom he resurrects as a vampire after she is killed by a Void attack, binding her to him as a servant. He also revives his loyal subordinates, the shadow assassins Shary and Blackas Shadow Prince, to aid him. He regards the girls of the 18th Platoon as his minions and becomes fiercely protective of them. Over the course of the series, Leonis develops a reluctant attachment to his new companions and begins to see them as more than mere tools, though he remains committed to his original mission.

As the former Undead King and strongest of the Dark Lords, Leonis possesses immense magical power, including mastery over demon swords, necromancy, and resurrection spells. His true form is that of a demon, but his current child body limits his full strength. He can still access his power when necessary, often using a cloak and mask to disguise himself as another Dark Lord named Zol Voldis. He also has the ability to turn others into vampires, as he does with Riselia. His swordsmanship, honed from his days as a hero, remains formidable. Throughout the story, he grows stronger as he regains his lost abilities and adapts to the new world, all while balancing his hidden agenda with the relationships he forms at the academy.