Leonis, whose full name is Leonis Death Magnus, is the main protagonist of The Demon Sword Master of Excalibur Academy. Originally known as the Undead King and the most powerful of the Dark Lords, he was a demon king who ruled over the nation of Necrozoa during the era of the Holy God Calendar one thousand years ago. In his past life before becoming a Dark Lord, he was an orphan who was discovered to possess the soul of a hero and was trained in swordsmanship, eventually rising to become the Hero of the Holy Sword and one of the Six Heroes. However, he was betrayed and assassinated by the nobles of his own kingdom. Nearing death, he was resurrected by a mysterious girl who transformed him into a Dark Lord, granting him new purpose. Devoted to her, he embraced the mission of overthrowing human society and became the strongest Dark Lord. When the remaining heroes set out to eliminate all Dark Lords, Leonis realized he could not defeat his former comrades and sealed himself inside a crystal coffin, choosing to reincarnate one thousand years later.

He awakens in the present day in the body of a ten-year-old human boy and is discovered by Riselia Ray Crystalia, a young girl from Excalibur Academy. Mistaking him for a refugee from the Void attacks, Riselia takes him into her care and enrolls him at the academy. Leonis initially intends to conquer the world and rebuild his demonic army, but he must first understand how the world has changed during his slumber. After a Void attack that costs Riselia her life, Leonis resurrects her as a vampire using his dark sorcery, binding her to him. He enrolls at Excalibur Academy under the name Leonis Magnus to gather information about the current world and to continue his hidden quest to find the vessel of the Goddess of Rebellion, Roselia, to whom he made a promise.

Leonis is a very responsible and mature person who is always clear about his objectives and does not hesitate to annihilate his enemies or anyone who stands in his way. Despite his ruthless approach to opposition, he shows a kind and protective side toward those he considers his subordinates. This contrast between his youthful childlike appearance and his commanding, ancient personality often draws attention. He is driven by his promise to Roselia and the mission she gave him. He enjoys coffee, claiming its color fits the demon army's theme, and dislikes eating vegetables, though Riselia forces him to consume them. He has a fondness for large breasts, particularly those of his 18th Platoon companions, and considers all the girls of the 18th Platoon as his subordinates, becoming angered when anyone tries to harm them.

His key relationships include Riselia Ray Crystalia, who becomes his vampire servant and the one he ultimately protects; Roselia, the Goddess of Rebellion, who gave him purpose; and the members of his 18th Platoon, including Regina Mercedes, Elfiné Phillet, Sakuya Sieglinde, and others. His two direct servants are Shary, a shadow assassin, and Blackas, known as the Shadow Prince. Leonis becomes popular among the girls at the academy, and he grows accustomed to sleeping next to Riselia, who often hugs him in her sleep.

Leonis develops over the course of the story as he adapts to the modern world, forms bonds with his platoon, and continues his secret mission to locate Roselia and rebuild his dark army. He displays overwhelming magical might and is considered the strongest of the Dark Lords. His notable abilities include the Demon Sword techniques and powerful dark magic, such as the Death Realm Sorcery, which he uses to resurrect Riselia as a vampire queen, marking her with his symbol of dominance. He can also disguise himself by donning a cloak and mask to assume the identity of another Dark Lord, Zol Voldis. Despite his young physical form, he retains his ancient knowledge and magical power, making him a formidable combatant.