She was contacted by Buffy's Scotland-based Scooby Gang (of Buffy, Willow, Xander, Dawn, Andrew, Renee and Satsu) after a cabal of Japanese vampires stole Buffy's scythe: the source of the Slayer powers after Willow's spell in "Chosen" activated all the potential Slayers in the world. Buffy, fearing these vampires - who possess unique magical abilities stolen from Dracula - play to reverse Willow's spell.
It was Aiko who discovered the Japanese vampire clan's leader's name: Toru. She trails the vampires, but they are aware of this and ambush her, and test their spell, succesfully removing her Slayer powers. Her lifeless body is left hanging from a building in Tokyo as a message to Buffy.