Beelzebub is a prominent deity from the Philistine and Abrahamic pantheons who serves as the gods representative in the eighth round of the Ragnarok tournament, where he is pitted against the human scientist Nikola Tesla. Known by a multitude of titles, he is called the Lord of the Flies by humans, while other gods refer to him as Anathema, meaning the one cursed by Satan. He is also known as the Dark God, the High Priest of the Deadly Sin of Gluttony, and the Ruler of Evil Spirits. In some traditions, he is worshipped as Baalzebul, a dual-faced deity representing both prosperity and pestilence.

Beelzebub's past is defined by immense tragedy and a profound, self-destructive curse. He was born with a dark entity or alter ego known as Satan residing within his soul. This condition, called Demonic Destrudo, triggers an uncontrollable and violent transformation whenever Beelzebub's love for someone reaches its absolute peak. In this state, he loses consciousness and is compelled to destroy the very individuals he cherishes. Initially unaware of this curse, Beelzebub formed deep bonds with three angels: Lucifer, Azazel, and Samael, who were unafraid of his ominous reputation. Their friendship ended in tragedy when Satan took control and murdered them. Later, while searching for the entity he believed responsible, he partnered with Lilith, a goddess and childhood friend of Lucifer. As they grew close, Beelzebub fell in love, which triggered the curse again, causing him to kill Lilith with his own hands. It was at this moment he realized that the Satan he sought for revenge was none other than himself.

As Lilith lay dying, she forgave Beelzebub and bestowed upon him a magical tattoo on his chest, known as Lilith's Mark. This mark has a unique and powerful effect: it prevents Beelzebub from dying by his own hand or deliberately allowing others to kill him, physically stopping any suicide attempt. Beelzebub initially viewed this as a curse forcing him to live in suffering, but Hades, the king of the underworld, later reinterpreted it as Lilith's prayer for him to continue living. Consumed by guilt and despair, Beelzebub became obsessed with finding a way to die. He decided that his only path to atonement was to live a miserable existence and eventually die a death that is painful, brutal, and ugly. This desire drives nearly all of his actions, from his horrific biological experiments to his participation in Ragnarok, where he hopes a human fighter might finally be capable of ending his life.

Beelzebub's personality is defined by a calm, collected, and deeply apathetic demeanor. He is a textbook example of a mad scientist, possessing a genius-level intellect that he applies without any ethical restraint. He keeps a private laboratory where he conducts gruesome experiments on living beings, treating them as mere subjects for his own curiosity and pleasure. This is most starkly illustrated when he cultivates the remains of the demon Hajun into a seed and implants it into the god Zerofuku, simply because the opportunity was convenient, leading to Zerofuku's complete consumption and death. He shows no remorse or guilt for such acts, viewing them as nothing more than data collection. Despite this cold cruelty, Beelzebub is capable of forming genuine bonds and loyalty. His relationship with Hades is the most significant example; after being defeated by him, Beelzebub came to deeply respect and trust the underworld king, who became his only emotional refuge. Hades gifted him his divine weapon, the Staff of Apomyios, a symbol of this trust. When Hades is killed by Qin Shi Huang in the seventh round, Beelzebub's grief transforms into a burning hatred for humanity, and he volunteers to fight in the next round specifically to avenge his fallen friend and exterminate the human race.

Throughout the story, Beelzebub remains a figure of isolation, shunned by most other gods who believe that associating with him brings disaster. He shows no fear when confronted by powerful deities like Odin, calmly brushing off his threats. His role in the tournament is pivotal. Before his own fight, he is revealed to be the mastermind behind the resurrection of Hajun, who terrorized the sixth round. When Hajun is defeated, Beelzebub burns his research on him, considering the experiment a failure because the demon could not kill him. After volunteering for the eighth round, Beelzebub faces Nikola Tesla in a battle of science versus divine power. During the fight, Beelzebub's character shows subtle development; he is forced to adapt to Tesla's innovative techniques, like the Gematria Zone and Tesla Warp, and for the first time, he witnesses the determination and will of a human fighting for his species. In the end, Beelzebub wins a hard-fought victory, killing Tesla. As he stands over his fallen opponent, hearing the cheers of the gods who now respect him, he comes to a quiet realization that gods and humans are not so different after all.

As a combatant, Beelzebub possesses abilities that make him one of the most dangerous gods in the tournament. He has all the standard divine attributes, including superhuman strength, speed, reflexes, and semi-immortality. His most notable power is his ability to manipulate high-frequency vibrations, an ability he calls Palmyra, or the Demonic Wingbeat. He has perfected this skill to an extraordinary degree, specializing his left hand for defensive vibrations and his right hand for offensive ones. When wielding the Staff of Apomyios in his left hand, he can project an almost indestructible vibrational barrier known as Sorath Samekh, or Gates of Hell, which is said to be capable of repelling even Thor's mighty hammer, Mjolnir. With his right hand and the staff, he unleashes devastating attacks like Sorath Vau, which takes the form of a giant fly and can overwhelm powerful strikes. His ultimate and most destructive technique is Original Sin of the Netherworld No.0 Chaos, a forbidden, self-destructive move where he forces his offensive and defensive vibrations to resonate together, creating a massive black sphere that ruptures everything inside it, causing immense damage to his opponent while also severely injuring himself. Beelzebub also possesses a fearsome transformation triggered by the Satanic curse, known as Demonic Destrudo, where his eyes turn black and his hands become claws, allowing him to tear through anything he loves with savage fury.