Belial, originally named Yukari Sanjo, is a founder and the supreme leader of the Kisaragi Corporation, overseeing warfare operations as one of its three top-ranking executives alongside Astaroth and Lilith.

Born into a venerable family, Yukari was once a kind, reliable, and well-mannered intellectual with black hair. She supported her childhood friend Astaroth in establishing the corporation, working closely with Lilith. After her subordinate Six was gravely injured in a Hero attack, Yukari underwent a series of enhancement surgeries performed by Lilith to acquire pyrokinesis and pursue vengeance. These procedures altered her appearance to a flame-haired demonic form and enhanced her physical capabilities, but also eroded her mental state, resulting in substantial memory loss and a profound shift in personality.

As Belial, she leads the combat division. Her transformation granted her red hair, an initially skimpy outfit later replaced by red power armor, and a commanding presence. She possesses the largest cup size in the organization.

Her demeanor became brutal, overbearing, inattentive, unreasonable, moody, rude, and impulsive. A tough and demanding workaholic, she frequently issues orders to subordinates, often threatening violence for disobedience or making capricious demands—such as instructing an employee to change their name and vocal tone for being too similar to her own. Despite these traits, she is regarded as a reliable and nurturing leader who demonstrates genuine care for her employees.

Belial holds the highest power level in Kisaragi Corporation due to undergoing the most enhancement surgeries. Her primary ability is pyrokinesis, enabling her to generate and control fire with a force comparable to ten thousand tons of TNT. Consuming nitro cartridges amplifies this power while producing an alcohol-like effect on her brain. She can also emit microwaves to heat the cells of living organisms, a lethal technique she prefers to avoid using.

She assigned Six to an exploration and conquest mission in an alternate world after losing a dice roll decision. During a peaceful period on Earth, she visited this fantasy world herself, aiding Six and other agents against an assault by the Hiiragi Tribe. She became lost in the Darkwood and eventually arrived in the Grunade Kingdom, where she assisted in battling the Primordial Dragon Midgard. This journey led to the recovery of her lost memories.

Her relationships are multifaceted. She considers Combat Agent Six a "stupid younger brother"; they once shared an apartment before her transformation, during which he often sexually harassed her. With Astaroth, her childhood friend and co-founder, she shares a bond marked by support and teasing, often mocking Astaroth’s crush on Six. Her professional relationship with Lilith is strained due to the memory loss caused by Lilith’s surgeries, though Lilith now works to help restore those memories. Belial also recruited Agents F18 and F19 into Kisaragi following a confrontation.

Her hobbies include riding motorcycles on long journeys.