One of the pair of twins of unknown, assumed Japanese origin, that were held captive and brutally trained to become members of the Adepts as children. While Sasha endured thanks to Olja's support, Yuu and Aoi had to rely upon each other, which in the end became their demise, culminated in Yuu apparently becoming a homicidal maniac and an extremely powerful Qwaser. His sister made contact with Sasha via a love letter, pleading him to find and stop her brother from his mad rampage, a request that he granted and that pitted him against his strongest enemy to date. Being metal manipulators, both Sasha and Ekaterina were initially at a huge disadvantage against Yuu, as he could simply oxidize whatever they threw at him at astonishing speed, fly and create vacuums or pressures of hundreds if not thousands of fathoms through his element, oxygen.
SpoilerOnly during the last confrontation between Sasha and Yuu it is learnt that he died long time ago, and that the one assaulting him was Aoi all the time: condemned to become a supply of Soma for the Adepts, she unconsciously awakened her powers as a Qwaser and obliterated her assailant, only to discover that it was her brother. This drove her to utter madness and created a split personality that emulated her dead brother, and it is with her last glimpse of sanity that she urged Sasha to kill her before her murderous instincts would take over again.