"Suikoden Demon Century" is set in a world where humans coexist with demons, though tensions between the two groups are high. The story centers on a young man named Kaito, who discovers he is the reincarnation of a legendary hero destined to unite humans and demons. This revelation comes after he encounters a mysterious demon named Lira, who reveals his true heritage and the existence of the "Demon Century," a prophecy foretelling a century-long conflict that will determine the fate of both species.

Kaito is drawn into a conflict involving the Demon King, a powerful entity seeking to dominate both humans and demons. Alongside Lira and a group of allies, including a human knight named Elena and a rogue demon named Zagan, Kaito embarks on a journey to gather the 108 Stars of Destiny—a group of individuals fated to play pivotal roles in the coming war. Each member of the 108 Stars possesses unique abilities and backgrounds, ranging from skilled warriors to cunning strategists, and their recruitment becomes a central focus of the narrative.

As Kaito and his companions travel across diverse regions, they face numerous challenges, including rival factions, political intrigue, and internal conflicts within their own group. The series explores themes of unity, prejudice, and the struggle for coexistence, as Kaito grapples with his dual identity as both human and demon. The Demon King's forces grow increasingly aggressive, leading to large-scale battles that test the resolve and loyalty of Kaito's allies.

The narrative builds toward a climactic confrontation between Kaito's coalition and the Demon King's army, with the fate of both species hanging in the balance. Throughout the series, character development is emphasized, particularly Kaito's growth from an uncertain youth to a decisive leader. The relationships between the main characters evolve, with alliances tested and betrayals revealed, adding layers of complexity to the overarching plot. The story concludes with a resolution that addresses the central conflict while leaving room for potential future developments.