Biagio Busoni is a character from the anime A Certain Magical Index II, serving as a prominent antagonist during a specific story arc. He is a high-ranking member of the Roman Catholic Church, holding the position of bishop. Physically, he is depicted as a middle-aged man in his forties, characterized by his blue eyes and blonde hair that reaches his neck. His appearance is notably ostentatious; he wears an elaborate white and gold robe and adorns himself with multiple pieces of golden jewelry. His most distinctive feature is the four necklaces he wears, which are wrapped around his neck like the rings of a tree trunk, from which dozens of small golden and silver crosses hang.

In terms of personality, Biagio Busoni is portrayed as an arrogant, ruthless, and fanatical individual. He holds the glory of the Roman Catholic Church above all else and is an extreme elitist, viewing those outside his faith with contempt, often referring to non-believers with derogatory terms. His cunning and manipulative nature are said to surpass even that of a cardinal. This arrogance is paired with a cruel streak; he is entirely willing to sacrifice the lives of his own subordinates to achieve his goals or preserve the Church's reputation, and he has little patience for those he deems incompetent, despite failing to effectively cultivate talent within his own ranks. His defining motivation is to eradicate the influence of Academy City and the entire Science Side, which he sees as a heretical threat to the supremacy of the Roman Catholic Church.

Within the narrative, Biagio Busoni is assigned as the commander and overseer of the "Queen of the Adriatic Sea" fleet, a massive magical weapon system. His primary mission is to complete a magical ritual known as the "Limited Cross" or "Rosary of the Limited Cross," which would remove the weapon's built-in restriction that only allows it to target the city of Venice. If successful, this would enable the fleet to attack and annihilate Academy City, effectively destroying all scientific progress and influence in one blow. He commands a fleet of magical ships and is also placed in charge of the Agnese Forces, a group of battle nuns who have been enslaved and forced to work on the flagship as punishment for a previous failure. His key relationships are largely adversarial. He directly clashes with the protagonist, Touma Kamijou, whom he underestimates. He also has a cruel, master-servant relationship with Agnese Sanctis, the leader of the Agnese Forces, whom he intends to use as a sacrifice for his ritual.

Over the course of his confrontation with Touma Kamijou and the Amakusa-style Church, Biagio Busoni experiences a definitive defeat. After a prolonged battle where he initially has the upper hand, his magic is overcome by Touma's Imagine Breaker, and he is physically defeated. Following his failure, he is captured by the English Puritan Church and imprisoned in the infamous Tower of London alongside another Roman Catholic conspirator, Lidvia Lorenzetti. He later appears during his imprisonment, where he endures sleep deprivation as an interrogation tactic. Even in this weakened state, his supreme arrogance and defiance remain intact, refusing to cooperate with his captors from Necessarius.

Biagio Busoni's notable abilities are entirely magical, centered on the many crucifixes he wears, which he collectively refers to as the "Menorah". His magic allows him to activate the various symbolic and historical meanings associated with the cross to launch powerful attacks. One technique, based on the legend of Saint Margaret, causes the small crosses to rapidly expand to the size of massive projectiles upon being thrown. Another, drawing from the weight of the cross as a symbol of humility, allows him to multiply a cross's weight by several thousand times, turning it into a high-speed, cannonball-like projectile. His most complex spell, based on Simon of Cyrene carrying the cross for Jesus, forces an enemy to bear the accumulated weight of all people and objects in the surrounding area, crushing them under a massive, invisible force. Through these abilities, he claims his personal power rivals that of an entire holy sanctuary, making him a formidable magician whose strength lies not only in raw force but also in his command over the fleet's coordinated attacks.