Regulus Corneas serves as a Sin Archbishop of the Witch Cult, embodying the sin of Greed. His history spans over a century, beginning in a small village outside the Four Great Nations. Upon receiving the Witch Factor of Greed, he annihilated his alcoholic father, complaining mother, brothers, his entire village, a neighboring town, and his home country. This destruction stemmed from his distorted perception of their kindness as condescending pity. His childhood love became his first wife after he slaughtered her family; her subsequent suicide traumatized him, leading him to demand expressionless compliance from all future wives. By the main story's events, he possessed over 200 wives, viewing them as possessions within his self-proclaimed "little kingdom."

Physically, Regulus appears as a pale-skinned young man with white hair and golden eyes that shift from disinterest to intensity during emotional outbursts. He typically wears white trousers, white shoes, a long-sleeved deep-blue dress shirt, and a pure-white greca-style coat featuring golden-edged cuffs, five golden buttons, and a high-neck collar bearing an infinity symbol. A blue teardrop-shaped earring hangs from his right earlobe. For his attempted wedding to Emilia, he adopted a formal white tuxedo with a cape-like back, maintaining his signature colors.

His personality is defined by profound hypocrisy and narcissism. He professes pacifism and selflessness while justifying mass slaughter as defense against perceived violations of his "rights." Interruptions or contradictions trigger violent rage, interpreted as personal assaults. An entrenched inferiority complex twists kindness into contempt, making him equate interaction with infringement. He delivers protracted, illogical monologues to assert dominance, masking deep-seated insecurities about pity or mockery. His warped concept of love reduces relationships to ownership, demanding wives embody purity and subservience without individuality.

Regulus wields the Authority of Greed, granting near-invincibility through two primary abilities. "Stillness of an Object's Time" halts time for himself, his clothing, or touched objects, rendering them immutable. This allows manipulation of air, water, or debris as unstoppable projectiles, ignoring gravity and negating durability-based defenses. "Lion's Heart" stops his heartbeat, halting biological processes and aging while conferring temporary invulnerability. Maintaining this beyond five seconds causes excruciating pain and risk of death. To circumvent this, "Little King" creates artificial hearts within his wives, tethering their vitality to sustain his invulnerability indefinitely. Destroying these hearts or eliminating all wives forces him back to the five-second limitation.

Notable conflicts include his early attack on Crusch Karsten's convoy with Lye Batenkaitos, massacring wounded soldiers while feigning victimhood. Roughly 100 years before the main story, he aided Pandora in assaulting Elior Forest, targeting the elf Fortuna as a bride and betraying Petelgeuse Romanée-Conti. During the siege of Watergate City Priestella, he abducted Emilia for marriage. His defeat resulted from Natsuki Subaru deducing his reliance on wives; after Emilia froze them, Subaru crushed Regulus's heart with the Unseen Hand, leaving him vulnerable. Reinhard van Astrea smashed him into the ground, and he drowned in the ensuing flood while raging against humanity.

Within the Witch Cult, Regulus acknowledged Pandora's superiority, addressing her respectfully despite his general arrogance. He despised Petelgeuse, dismissing him as "abominable" and facilitating his betrayal at Elior Forest. His fixation on Emilia was purely superficial, driven by her appearance as a bride candidate.

His worldview remained unchanged until death. Final moments revealed obsessive hatred toward perceived mockery and an unyielding conviction in his own righteousness, epitomizing his century-long evasion of accountability.