Fuegoleon Vermillion is the first son of the House Vermillion, one of the royal families of the Clover Kingdom, and serves as the captain of the Magic Knight squad known as the Crimson Lion Kings. He is a tall man with straight vermillion hair, purple eyes, and distinctive red markings around his eyes and a diamond-shaped mark on his forehead. His typical attire consists of a dark blue high-collared shirt, a purple sash, white trousers, and a long red cape bearing his squad's insignia.

As a person, Fuegoleon is defined by a strong and fair sense of justice. He judges people based on their true worth and character rather than their social status, which is an uncommon trait among the nobility of the Clover Kingdom. This is demonstrated when he defends Asta, a commoner without magic, from oppression by the noble House Silva, arguing that the Wizard King himself has acknowledged the boy's potential. Fuegoleon possesses a charismatic and calm leadership style, allowing him to quickly assess dire situations and deploy orders effectively. He is also a wise and no-nonsense mentor, choosing to scold those who lose focus while also offering crucial encouragement, such as when he advises Noelle Silva to overcome her self-doubt during battle. Despite this open-mindedness, he retains the pride of a noble and is easily angered by condescension or impoliteness from peers like Nozel Silva. Yami Sukehiro has affectionately nicknamed him the hot-blooded seriousness king.

Fuegoleon's primary motivation is his ambition to become the Wizard King, a goal he has held since childhood. This vow is so significant that he marked himself with the diamond-shaped symbol on his forehead to represent its weight within his family. His role in the story is that of a powerful and respected figure who serves as a mentor and a benchmark for younger characters, particularly Asta, Noelle Silva, and his own younger brother, Leopold Vermillion. He forges a strong rivalry with Nozel Silva of the Silver Eagles, a relationship that persists from their youth into their positions as captains. He shares a deep and unique bond with his older sister, Mereoleona Vermillion, who temporarily replaces him as captain after he is incapacitated. Their intense relationship includes a childhood fight known as Hot-Blooded Tuesday, which destroyed part of their family estate.

Fuegoleon undergoes a significant development arc following a surprise attack by the Eye of the Midnight Sun. During this encounter, he loses his right arm and falls into a prolonged coma after being betrayed and overpowered by William Vangeance's imposter, Patolli. He remains in this state for a considerable period, during which his sister Mereoleona takes command of his squad. He awakens during a later invasion by reincarnated elves, having been chosen by Salamander, the great spirit of fire. This bond rejects the elf spirit inhabiting his body and restores him, granting him a new arm made of fire and significantly enhancing his magical power. After recovering, he returns to his position as captain, more powerful than before, and fights to protect the kingdom.

As a royal, Fuegoleon possesses immense natural magical power. His magic attribute is fire, which he wields with both raw, overwhelming force and precise, tactical finesse. Among his notable fire creation spells are Leo Rugiens, which creates a giant lion of flames to burn everything around it, and Ignis Columna, which surrounds him with a defensive vortex of flames. He is also capable of using Leo Palma, creating hands of flame lions to seal an opponent's movements. After forming a pact with Salamander, he gains access to powerful spirit magic, with his ultimate technique being Salamander's Breath, a devastating blast of fire capable of incapacitating extremely powerful foes. His already formidable abilities, combined with his noble spirit and leadership, make him one of the most powerful and respected Magic Knights in the Clover Kingdom.