Marquis de Léon is a lion-like monster with the ability to move twice per round, casting Sizzle and breathing frost. He must be fought twice, once each in the fourth and fifth chapters. It is supposed to be impossible to defeat him in the fourth chapter; if he is defeated in the FC/NES version, the battle will repeat until the player loses. In the subsequent remakes, Marquis de Léon has unlimited HP.
Not much is known about him as a human, although it is implied that he struck a deal with Psaro the Manslayer which led to him and his entire kingdom being brainwashed into servitude (likely a result of naivety and lack of common sense). As a monster, Marquis de Léon was a brutal tyrant, striking the fear of all citizens in not only his kingdom but the entire continent. It is known that several unfortunate citizens were locked up in prison simply for displeasing him. While he worked alongside Balzack, the two did not appear to work very well together, as the Marquis admonished the latter for losing a battle and sent him away to another kingdom when given the chance. When two of his prisoners escaped prison and fled the continent, Marquis de Léon no longer allowed any form of immigration off the continent by banning all ship transportations.
Once the prince of Palais de Léon and the son of the previous king, Marquis de Léon is now a servant of Psaro the Manslayer. Having used the Secret of Evolution to transform into a lion-like monster, he imprisoned his father and took the throne for himself, claiming that his father passed away.
In Chapter 4, Meena, Maya and Oojam infiltrate the palace in search for Balzack who murdered the former two's father. They confront and defeat him in the hidden throne room. Having witnessed this (while not being in either sisters' sight), Marquis de Léon reprimands Balzack for his failure and proceeds to defeat the three himself with ease. The three are then imprisoned in the same cell as the old king, who help them escape.
Much later in Chapter 5, Marquis de Léon once again encounters the sisters, this time with the Legendary Hero in tow. By this point, the Marquis already sent Balzack away to Zamoksva, no longer requiring his help. He then once again engages the heroes in combat, resulting in his defeat.
Marquis de Léon's fate differs between the originals and the remakes. In the NES version, he dies after his defeat. In the remakes, he turns back to his human form, having no memory of his servitude to Psaro. This implies that he was brainwashed into serving the Master of Monsters rather than through his own free will. According to an NPC in Havre Léon, the prince made a deal with the devil in exchange for his soul, which may be the cause of his mind-control. Upon his defeat, the entire kingdom wake up from a trance, meaning that everyone was under Psaro's spell rather than just the Marquis.