Kiyoshi Mitarai, code named Seaman, is a member of the Sensui Seven and a major antagonist in the "Chapter Black" arc of Yu Yu Hakusho.
While heading home from a Megallica concert, Kuwabara and his friends were attacked Seaman, who took notice of Kuwabara's current lack of spirit energy and decided take the opportunity to take him out. Using his water manipulation powers, Seaman trapped Kuwabara and his friends inside a water giant he created. However, Kuwabara suddenly utilizes his spirit energy to create a spirit sword, which he uses to slice through the water giant and free himself and his friends. While overpowering Seaman, Kuwabara spares his life and takes him to safety. Kuwabara brings his unconscious body to Yusuke's apartment, where he awakens. He is then interrogated by Team Urameshi and informs the team about the Chapter Black tape and Sensui's plans.
During the confrontation between Yusuke and his friends and Sensui, Seaman stays in an apartment with Botan and watches the battle. When an explosion in the apartment building causes furniture to fall over, Botan shields Seaman from a falling bookcase, confusing Mitarai as to why she would do something like that for him. Seaman then begins to realize that humans were not all evil as he had once and become more optimistic on his outlook towards them. Seaman later helps Team Urameshi get to Sensui's lair and defeat another one of Sensui's minions, Tsukihito Amanuma aka the Game Master. After Sensui is defeated, Seaman returns to middle school and later gets accepted into the city's finest high school.