High Priestess Bishumu is one of the three High Priests of the dark organization Gorgom, serving directly under the Creation King. She is the only female member of this ruling triumvirate, which also includes her colleagues Darom and Baraom. As a High Priestess, she possesses the Stone of Earth, a source of her power and a key component in Gorgom's plans.

Bishumu is an ancient being, having lived for thousands of years. In her standard priestess form, she has a relatively human-like appearance compared to her peers, characterized by a pale, almost white outfit and hood. She can discharge powerful beams of searing heat from her eyes, which are also said to have the ability to see both the past and the future. In terms of personality, she is cunning, overconfident, and ruthless. While appearing elegant and sophisticated, she is冷酷 (reikoku) or cruel, often favoring psychological attacks that exploit the weaknesses of human hearts and emotions, particularly targeting young people. Despite her cruelty towards humans, she is shown to have a sense of loyalty and care for her fellow Gorgom monsters.

Bishumu's primary motivation is the success of Gorgom and, to an extent, her own ambition. As the series progresses, she becomes particularly focused on proving herself, often ignoring the more cautious advice of Darom and Baraom. A significant part of her motivation is her desire to become the consort of Shadow Moon, the newly awakened creation king. Her actions in the latter part of the story are heavily influenced by a desire to win his favor and stand by his side.

Her role in the story evolves as she dedicates her power to a greater cause. Bishumu uses the energy of her Stone of Earth to help revive Shadow Moon. This act depletes her power, forcing her to be reborn into a new, more monstrous form known as Grand Mutant Bishumu. In this form, she resembles a white-haired, purple pterosaur-like creature, gaining the ability to fly and to spin her body rapidly to create powerful tornadoes. She and the other two priests, now also reborn as Grand Mutants, serve Shadow Moon directly and engage in more frequent and direct battles with Kamen Rider BLACK.

Her key relationships are primarily with her fellow priests and with Shadow Moon. She works alongside Darom and Baraom, though she often displays a rivalry with them, believing her strategies to be superior. Her relationship with Shadow Moon is one of devoted service mixed with romantic aspiration, which ultimately leads to her downfall.

Bishumu's development culminates in her final confrontation with Kamen Rider BLACK. Driven by overconfidence and a desire to finally defeat him, she kidnaps Kyoko Akizuki, the sister of Shadow Moon, to use as a hostage to force Kotaro Minami (Kamen Rider BLACK) to surrender his Kingstone. During the ensuing battle, she physically restrains Kamen Rider BLACK and calls upon Shadow Moon to fire a powerful beam that would destroy them both, ensuring the hero's death. However, when Kyoko rushes to shield Kotaro, Shadow Moon hesitates, his lingering affection for his sister causing the beam to veer off target. As a result, Bishumu alone is struck by the full force of the attack. Confused and enraged, she questions Shadow Moon's betrayal before exploding and being destroyed. Her notable abilities include firing laser beams from her eyes in her priestess form, and in her Grand Mutant form, flight and the generation of powerful twisters.