Yuu Kuchiba  

, 朽葉 悠

One of the two siblings, believed to be of Japanese descent, was kidnapped and trained by the Adepts as children. Although Sasha received support from Oljas, Yuu and Aoi had to depend on each other, which ultimately led to their downfall. It appears that Yuu became a violent Qwaser, while his sister contacted Sasha through a letter asking him to stop her brother's destructive behavior. Sasha granted the request and faced off against his strongest enemy yet. As metal manipulators, both Sasha and Ekaterina were at a significant disadvantage compared to Yuu, who could rapidly oxidize any substance he encountered, fly, create powerful vacuums or pressures of hundreds or thousands of fathoms through oxygen, and utilize his element in numerous ways.