Katsuhiko Jinnai is the primary antagonist and a student at Shinonome High School, where he serves as the student council president. Before being transported to the alternate world, his political career is jeopardized when it is discovered that he illegally used student government funds to bribe campus clubs for votes in his election. Facing impeachment, his plans for a counterattack are interrupted when he, along with his rival Makoto Mizuhara, his sister Nanami, and teacher Masamichi Fujisawa, is pulled into the world of El-Hazard.

Within El-Hazard, Jinnai receives a unique ability as a side effect of his transportation: the power to communicate with the Bugrom, a race of human-sized insectoid beings. He quickly uses this gift to become their military commander, earning the favor of their queen, Diva, who believes he is a divine messenger sent to lead them to victory. As the Bugrom’s general, he reorganizes their forces into an effective army, successfully conquering much of the land and coming close to total victory over the human nations.

A power-hungry and ruthless individual, Jinnai is also a skilled tactician and a multifaceted manipulator. His defining trait is an obsessive and psychotic rivalry with Makoto Mizuhara. He blames Makoto for every failure and disappointment in his life, from academics to athletics, and views him as his lifelong enemy. His primary motivation for conquering El-Hazard is not just power for its own sake, but to gain the ability to finally best, humiliate, or even kill his rival. He possesses an enormous ego, insisting on being called Lord God Jinnai, and revels in cruelty, yet he also demonstrates an unexpected level of resilience, quickly bouncing back from any defeat.

Beyond his Bugrom communication ability, Jinnai is a cunning strategist and a tenacious schemer. His role in the story is to provide the primary source of conflict for Makoto and the other protagonists as they navigate El-Hazard. He is a surprisingly complex figure who brings dramatic tension to the series. In his quest for power, he seeks out the legendary Demon God, Ifurita, and briefly becomes her master. Their relationship is notable; although he initially attempts to use her as a weapon for conquest, he eventually develops a degree of fatherly affection and care for her, revealing a softer side to his otherwise megalomaniacal personality. In a final act that is not entirely altruistic, he aids Makoto and the others in stopping a greater catastrophe, believing that a true ruler does not destroy the land he wishes to conquer.