Belphegor, nicknamed Belphie, ranks as the youngest among the seven demon brothers and embodies the Avatar of Sloth. Originally an angel in the Celestial Realm, he maintained a close bond with his twin brother Beelzebub and younger sister Lilith. His repeated journeys to the human world ignited Lilith's curiosity about humans, leading to her falling for one. When punished for attempting to heal her human lover, Lilith's plight drove Belphegor and his brothers to rebel against their father. During the Great Celestial War, Beelzebub chose to save Belphegor over the fatally injured Lilith, an event that left Belphegor profoundly scarred.

After their transformation into demons and relocation to the Devildom, Belphegor cultivated a fierce hatred toward humans, holding them accountable for Lilith's death. This resentment intensified when Lord Diavolo initiated an inter-realm exchange program. Belphegor vehemently opposed the project and begged Lucifer to terminate it. After Lucifer dismissed his fears about Lilith, Belphegor threatened humanity's annihilation. Lucifer overpowered him, imprisoning him in the House of Lamentation's attic under a magical seal only angels or humans could break.

While captive, Belphegor deceived the human exchange student (MC) by masquerading as a fellow prisoner. He urged them to form pacts with all six brothers to dismantle the attic's seal. Upon discovering his true identity and freeing him, MC faced Belphegor's wrath—he attacked and mortally wounded them. Learning later that Lilith had survived and that MC descended from her, Belphegor relinquished his hatred, reconciled with his brothers, and forged a protective connection with MC. In the spin-off "A Devilishly Beautiful Wedding," he replicates this arc with Eli, ultimately accepting her as kin.

Physically, Belphegor possesses dark indigo-grey hair with white tips, long bangs obscuring his right eye, and violet eyes featuring a pink gradient. His demon form displays twisted ram horns, a thorned cow-like tail, and cow-spotted birthmarks on his neck and right ankle. He consistently wears cerulean blue nail polish. His attire frequently incorporates cow motifs, such as a black hoodie with a horseshoe pattern, harem pants, and combat boots. Casual wear includes a cow-patterned pillow and an asymmetrical windbreaker.

Belphegor epitomizes Sloth through lethargy and profound despair (historically called "Acedia"). He avoids exertion, detesting chores or repetitive tasks, and prioritizes naps as his "daily section." Beneath this apathy lies manipulation, defiance of authority, and repressed fury over Lilith's loss. His despair fuels emotional detachment, rendering anger his most accessible emotion. Despite a languid exterior, he acts swiftly when motivated. He expresses remorse through actions rather than words and exhibits jealousy and possessiveness in relationships, striving to "bind and monopolize" loved ones while neglecting anniversaries.

His preferences encompass "chill" piano melodies, white chocolate, sushi, Quetzalcoatl brains, and stargazing. He professes to like cats but favors dogs. He enjoys thrill rides, horror films, and dolphins. He avoids celery, apples, baby talk, and disrupted sleep. Notably, he bathes by washing his hair first to prevent confusion.

Relationships shape his evolution. He shares a telepathic link with Beelzebub, assisting him despite his slothful nature. His rift with Lucifer persists post-imprisonment, though latent concern remains. He collaborates with Satan's "Anti-Lucifer League" for pranks yet seeks Lucifer's counsel. His hostility toward Diavolo wanes after an apology. With MC or Eli, he transitions from manipulation to steadfast loyalty, messaging them constantly and imagining future travels alongside Beel and them.

Trivia notes his lack of artistic talent, habitual texting to Eli, and historical alliance with Asmodeus during their angelic era. Demonological lore emphasizes his cunning and manipulative traits.