Bouichirou Yamato is a character from the anime Demon King Daimao who serves as a significant antagonist before later becoming a supporting figure. He is a time traveler who originates from hundreds of years in the future, having journeyed to the past using technology he himself created to break through the barriers of time. His physical appearance is that of an adult male with green eyes and blond hair that reaches his neck, often tied back in a ponytail.

In terms of personality and motivations, Yamato is a highly intelligent and driven individual. His primary goal is to alter the fundamental relationship between gods and humanity. He seeks to create a world where neither side has to rely on the other, thereby freeing the gods from the restrictive system imposed upon them by the High Priests. His actions are not driven by malice but by a complex and pragmatic desire to reshape the world's order. He holds a cynical view of human nature and free will, believing that happiness is merely a matter of biological satisfaction and that narratives and values are like viruses that infect sentient beings. He has a particular interest in Keena Soga, referring to her as the "principle of identity," and seeks to use her body to control the gods. Due to his connection with a character named Rimu, he had been waiting for Keena since her birth and was the one who gave her her signature hair decoration.

Yamato holds a high position in the government of his time, serving as the Director of the Cabinet Intelligence and Magic Office and as a member of the Imperial Public Safety Committee. His role in the story is initially that of a formidable antagonist. He is shown to be incredibly powerful, easily overpowering the protagonist Akuto Sai in their early encounters and demonstrating strength that surpasses Akuto's at that point in the narrative. He seeks to bring down Akuto for what he believes is the good of the world, willing to use any means necessary to achieve his ends. Over a century before the main events of the story, he also sealed the demon Peterhausen and provided the Brave suit to Hiroshi Miwa's grandfather.

Key relationships define much of his history. He is a mentor or figure of importance to characters like Hiroshi Miwa, and his long-term plan involving Keena Soga is central to his actions. His understanding of the world is shaped by a guidance teacher who taught him about the Law of Identity and the ancient God of the Old Planet's wish to destroy mankind. Throughout the series, Yamato undergoes development, transitioning from a primary antagonist to a supporting character as the narrative progresses and the true nature of the conflict becomes more apparent.

Regarding his abilities, Yamato is an extraordinarily skilled and powerful magic user. He is a master swordsman with centuries of combat experience. His signature technique is called the Hogson-style perfect slash, a sword technique that can ignore distance, cut with precision at a molecular level, and bypass any barrier by cutting through the dimension itself. He possesses the ability to control space and time, including teleportation, dimensional storage, and true time travel to any point in history. He can also fly using his sword, send targets to other dimensions, and has displayed superhuman physical characteristics and longevity, having looked the same for over a hundred years. His intelligence is extraordinary; he is the inventor of the space-time control device and the Brave suit.