Aiko was a Japanese Slayer. She led the Slayer team based in Tokyo.
Aiko was contacted by Buffy Summers after a group of Japanese vampires stole Buffy's scythe: the source of the New Slayers powers after Willow Rosenberg's spell activated all the potential Slayers in the world. She discovered the identity of the vampire Toru by defeating a group of Kabuki Demons by herself.
Aiko tracked down Toru and his right hand, Raidon through the streets of Tokyo. However, the two vampires managed to detect her and set a trap to capture her. Using an amulet created by the vampire witch Kumiko Ishihara, Toru depowered Aiko, turning her back into a regular girl. Toru broke Aiko's jaw and bit her, killing the former Slayer in the process. Afterward, Toru tied Aiko's corpse to a building and hung a banner on which he wrote "Welcome to Tokyo" using Aiko's own blood as a warning sign for Buffy and her team of Slayers.