Birugenia, known as the Sword Saint, is a formidable antagonist from the original Kamen Rider BLACK series. His origins are deeply tied to the evil organization Gorgom, as he was born on the day of a solar eclipse thirty thousand years before the main events of the story, the same celestial condition that marked the birth of the Century Kings Kotaro Minami and Nobuhiko Akizuki. Despite being born under the required omen, Birugenia was never granted a Kingstone by the Creation King and thus could not become a candidate for the next ruler of Gorgom. Enraged by this denial, he attempted to revolt but was defeated and subsequently sealed away in a coffin for millennia.

His release marks a turning point in the narrative. After the three High Priests of Gorgom repeatedly fail to retrieve the Kingstone from Kamen Rider Black, the Creation King orders them to unseal Birugenia to serve as their ultimate warrior. From his first appearance, Birugenia establishes himself as a threat not only to the hero but also to his nominal allies. His personality is defined by arrogance, selfishness, and a brutal sense of superiority. He holds the High Priests in contempt, frequently interfering with their plans and mocking their failures, showing no hesitation in assaulting them or using lower-ranking monsters as disposable tools for his own ends.

Birugenia's primary motivation is a deep-seated ambition to become the next Creation King himself. He believes that by killing Kamen Rider Black and claiming his Kingstone, he can finally seize the power that was unjustly denied to him. This obsession drives him to constantly seek a direct confrontation with Black, whom he views as his personal prey. Initially, his power is nearly unmatched, and he overwhelms Black in their early encounters, demonstrating a level of strength and combat skill that forces the hero to retreat or rely on desperate measures.

In terms of abilities, Birugenia is a master swordsman, wielding the Bilsaber and the Biltector, a sword and shield combination that makes him a formidable opponent in close-quarters combat. He possesses a wide array of special techniques. With his sword, he can generate Dark Storm, a gust of wind reaching speeds of two hundred meters per second, and fire energy waves called Pressure Wave. He can also use Demon Trick to create illusory copies of himself to confuse his enemy. Beyond his swordplay, Birugenia has other supernatural skills, such as the Face-Stealing Technique to assume the identity of others and the ability to control anyone who wields his sword. As the story progresses, he becomes even more powerful, reverting to an original white-faced form that enhances his abilities and arrogance. He also acquires the Hellshooter, a powerful motorcycle built to rival Black's Road Sector, and later receives the legendary Satansaber from the Creation King, allowing him to perform a devastating beam attack called the Satan Cross.

The character's role in the story evolves as his rivalry with Black reaches its peak. However, his greatest downfall comes not from his intended prey but from the awakening of the true Century King, Shadow Moon. After Shadow Moon is revived, Birugenia is quickly rendered obsolete. In a final confrontation at the Gorgom shrine, Shadow Moon disarms and kills him with ease. In his dying moments, Birugenia realizes that he was merely a tool used by the Creation King to provoke events leading to Shadow Moon's birth, a realization that fills him with despair as he perishes.