Arai Seikū is the son of Kenshin's reverse-blade sword maker, Arai Shakkū. In defiance to his father creating swords that kill, Seikū abandons the trade of forging swords and instead decides to earn a living through the sale of kitchen wares and knives that he manufactures.
However, after Himura Kenshin's Sakabatō breaks at the hands of Seta Sōjirō in the Shingetsu village, the former seeks Seikū's help in getting a new sword, in order to continue his battle against Shishio and his forces. Seikū respectfully refuses, however, Shishio's spies become aware of what has taken place and a skilled swordsman of the Juppongatana, named Sawagejō Chō, attains Shishio's permission to pursue Seikū, so that he can get his hands on the final sword manufactured by Shakku, which is at the time kept in a Holy Shrine (being the only sword that has not been bloodied).
As Chou makes his way to the shrine (which he learns of after threatening to kill Seikū's infant child, Iori, whom he takes hostage), he comes across Kenshin, who is forced to battle him using the his broken Sakabatō. Although Kenshin performs well and is able to down Chou several times, he finds himself at a major disadvantage facing Chou's Hakujin no Tachi. Finally, when Seikū, his wife, Makimachi Misao and Kashiwazaki Nenji come to the shrine, Seikū reluctantly decides to enter the shrine and retrieve his father's final sword, which he gives to Kenshin (knowing that Kenshin is putting his life on the line to protect Iori). With the new sword, Kenshin easily defeats Chou, believing briefly that he has broken his vow to kill. It is then discovered that the final sword was actually the principal forge of the Sakabatō (far more powerful than the one broken by Sojiro) and that Chou is still alive, though badly injured and later jailed.
He was portrayed by Dai Watanabe in the live-action sequel film.