Kiyoshi Mitarai, also known by the code name Seaman, is a secondary character introduced during the Chapter Black saga. He is a human teenager of about fourteen or fifteen years old, with a slender build, fair complexion, and delicate facial features. He has sandy brown or blond wavy hair that is typically slicked back with a few bangs hanging onto his forehead, and blue eyes. His typical attire includes a yellow hooded sweatshirt, tight blue jeans, and white shoes, though he sometimes wears a school uniform.

Before acquiring his powers, Mitarai was an ordinary middle school student living with his parents and older sister. He had a sincere but timid, introverted, and mentally vulnerable personality. A self-described social outcast, he experienced bullying at the hands of his classmates, which contributed to his feelings of isolation and low self-worth.

Mitarai was recruited into the group known as Sensui’s Seven by the rogue Spirit Detective Shinobu Sensui after the barrier between the human and demon worlds began to weaken, which caused his psychic abilities to awaken. After being shown the Chapter Black tape, a video compilation of humanity’s worst atrocities, he became deeply traumatized and was brainwashed into adopting Sensui’s misanthropic worldview. He developed a strong hatred for humans and desired to punish them for their immorality, though in doing so he became something of a hypocrite by turning into the kind of ruthless killer he claimed to despise.

In the story, Mitarai first appears attacking Kuwabara and his friends after a concert. Taking advantage of Kuwabara’s temporarily depleted spirit energy, he traps the group inside a giant water construct that functions as a dimensional prison. This act is part of Sensui’s plan to force Kuwabara to develop his latent ability to cut through dimensions. After Kuwabara breaks free using a newly formed Dimension Sword, he defeats Mitarai but chooses to spare his life and carries him back to safety, an act of mercy that leaves the confused teenager questioning his beliefs about human nature.

Later, while under the care of Yusuke’s group, Mitarai is protected by Botan when a falling bookcase nearly crushes him during an explosion. When she is wounded while shielding him, he is once again baffled by this selfless act, and his outlook on humanity begins to genuinely change. Realizing that not all humans are the ruthless killers he once believed them to be, he decides to assist Yusuke’s team. He leads them into Sensui’s lair and helps the group defeat another of Sensui’s psychics, the Game Master, before the final confrontation.

Mitarai’s primary ability is water manipulation, a power he calls Seaman. By mixing his own blood with water, he can control the liquid and form it into various creatures. The more blood used, the larger and more powerful the construct becomes. The most dangerous aspect of his power is that the interior of his water creatures exists in a separate dimension, making it nearly impossible for an opponent to break free from the inside unless they possess the ability to cut through dimensional barriers. Victims trapped inside are at risk of drowning.

After the defeat of Sensui, Mitarai returns to a normal life, returning to middle school. Having overcome his inner conflict, he gains confidence and an optimistic view of the future. He studies hard and is accepted into the city’s finest high school. In some accounts, he later studies abroad and becomes involved in environmental issues and international volunteer work.