Kakyou Kazuki is a character who exists in a perpetual state of dreaming, confined to a hospital bed in a heavily secured room and kept alive by life support systems. He is a yumemi, or dreamseer, a rare individual with the power to perceive the future through his dreams. This ability manifested at an early age, leading to him being kept as a prisoner by an unknown political group or government agents who sought to control his gift. His physical condition is a result of being shot while attempting to escape his confinement. The attempt was made in a desperate and futile effort to save the woman he loved, Hokuto Sumeragi, whose death he had already foreseen in his visions. Trapped in his own body and unable to take his own life, his greatest wish is to finally die so he can be reunited with Hokuto beyond the waking world.

Kakyou is a tall man with a perpetually sad expression, characterized by his long, pale hair and golden eyes. He is almost always dressed in white. His personality is quiet, polite, and soft-spoken, often coming across as passive and deeply melancholic. He shows little reaction to the immediate events around him, a detachment born from his long-held belief that the future is immutable and cannot be changed, no matter how much suffering it causes. This fatalism masks a profound and suicidal depression. The few moments of genuine emotion he exhibits occur in the dreamscape, particularly when he interacts with others like Kotori Monou or the spirit of Hokuto. Despite his nihilistic outlook, he is not without empathy; he feels guilt for his role in the coming destruction and shows genuine kindness to Kotori, helping her to fulfill her final wish.

In the conflict for the fate of the Earth, Kakyou is aligned as a Dragon of Earth, one of the Seven Angels who seek to destroy humanity to save the planet. He accepts this position not out of conviction for their cause, but out of a personal bargain. Fuuma Monou, the leader of the Dragons of Earth, promises to grant Kakyou his desire for death in exchange for his loyalty and service as a seer. Kakyou reluctantly agrees, becoming a tool for Fuuma, who manipulates him and others, including the dreamseer Princess Hinoto, by using him to corrupt her prophetic visions. He also appears in the dreams of the story's central protagonist, Kamui Shirou, sometimes to guide him and other times as a result of his servitude to Fuuma.

His most significant relationships are forged in the realm of dreams. His lost love, Hokuto Sumeragi, is the defining figure of his past. It was by gazing into her dreams that he saw the outside world, including the sea she showed him, and fell in love with her. Her death shattered his will, and his desire to join her is his primary motivation. He later forms a connection with Kotori Monou, a fledgling dreamseer. He recognizes her burgeoning power and the tragedy that awaits her, and he acts as a mentor and comfort to her. When Kotori is killed, Kakyou uses his ability to enter her dying dream and possesses her corpse, using it to hold the Sacred Sword in place to protect Kamui. He also shares a complex, manipulative relationship with Fuuma, who consoles him in his despair while also making him a tool for his plans.

Throughout the story, Kakyou undergoes a subtle but crucial development. Initially resigned to a future he believes is fixed, his interactions with Kotori and the lingering memory of Hokuto gradually plant a seed of doubt. This change culminates near the end of the series when, after receiving encouragement from Hokuto and Kotori, he finally acts on the belief that the future might not be set in stone. He rebels against Fuuma by enabling Hokuto to enter her twin brother Subaru Sumeragi's dream, convincing Subaru to aid Kamui in a pivotal moment. This act of defiance helps to shift the course of destiny. Ultimately, he achieves his peace. With the future now changed, his spirit is finally free, and he reunites with Hokuto, leaving his comatose body behind for the "outside" he had only ever seen in dreams.

Kakyou's abilities as a dreamseer are extensive. He can see glimpses of the future in his own dreams, and his visions are consistently accurate. He can also enter the dreams of others at will, communicate with them, and even draw waking people into his own dreamscape or manipulate their perceptions. This power extends to the point where he can inhabit the corpse of a recently deceased dreamseer during their final dream, as he did with Kotori. Despite his physical helplessness, his mastery over the dream world makes him a potent force in the spiritual battle that determines humanity's fate.