Kai Kudou is the central protagonist of the story. He is a teenage boy who, along with his younger sister Hikaru, was orphaned before the start of the series. His only remaining family is Hikaru, who is noted to be gravely ill with a condition that has never improved. He has brown hair and brown eyes, and is known to be extremely sensitive to sweets, to the point where even the sight or mention of them makes him feel uneasy.

Kai possesses strong psychic abilities, which in the world of the story classifies him as an Esper, or an E. His specific powers are not always clearly defined, but he is capable of telekinesis and generating powerful psychic blasts. There is a belief held by Eiji Sagimiya, the head of the Ashurum corporation, that Kai has the potential to unlock the same kind of immense, dormant power that his sister Hikaru possesses, though Kai is either unaware of this potential or is actively suppressing it.

In terms of personality, Kai is characterized by a deep-seated sense of ethics and a strong respect for life. He is fundamentally a good-natured and kind individual, which other espers are particularly able to sense about him. Despite the power he commands, he often comes across as humble and somewhat shy. He is driven by a profound need to protect his sister, and this motivation dictates most of his major decisions. To ensure Hikaru receives care for her illness and protection from a society that fears espers, Kai agrees to join the Ashurum corporation. As part of this arrangement, Hikaru is kept in an Ashurum hospital, while Kai is conscripted into a special psychic division called AESES, where he undergoes intensive training in combat, hacking, and developing his abilities.

Kai's role in the story is one of a soldier who gradually becomes a renegade. After a year with Ashurum, he is sent on a mission to Gald City to stop a guerilla group that is reportedly using psychics against their will. The mission goes horribly wrong when a civilian, terrified of espers, shoots and kills Kai's partner. A subsequent confrontation with his rival, Shen-long Belvedere, results in a massive psychic explosion that devastates part of the city. Kai is thrown into the sea and washes up on the shore of Gald, where he is rescued by a young girl named Asuka and her adoptive brother, Yuuki Tokugawa. As he recovers, Kai decides to stay in Gald with them to see the world outside Ashurum for the first time.

This period marks the beginning of Kai's major development. Away from the organization, he begins to learn the truth about Ashurum, discovering that it has been brainwashing its agents, including himself, to make them more powerful and compliant. He also starts to see the kinder side of humanity through his relationships with the residents of Gald, including Yuuki, who initially distrusts Kai because Yuuki hates espers, and Asuka, a young empath who develops a fondness for him. Kai's key relationships are central to his journey. His bond with his ill sister Hikaru is his primary drive. His relationship with Shen-long is complex, beginning as a rivalry fueled by Shen-long's jealousy over the attention Kai receives from Eiji, but eventually evolving into an alliance. His friendship with Yuuki and Asuka provides him with a new perspective and a reason to fight beyond mere survival.

Ultimately, after learning of Ashurum's true goal to use Hikaru to destroy the human race and speed up evolution so that only espers survive, Kai confronts Eiji to rescue her. He is aided in this final battle by Yuuki and his former rival, Shen-long. The outcome of this decisive battle for the fate of the world is not explicitly shown, leaving it ambiguous whether Kai survives or sacrifices himself to defeat Eiji.