Yuu is Nene Amano's younger brother who was enticed by DarkKnightmon with the Darkness Loader.
In reality, Yuu is a willing member of the Bagra Army who is both the previous user of Nene's Xros Loader and the eighth Death General. Having been against playing with others in fear of hurting them, he was deceived by DarkKnightmon under the notion that the Digital World is just a game and destroying Digimon and humans alike have no real consequences. As a result, Yuu became a sadist who fights with no holds barred even against his own sister. During the events in the Hell's Field battleground under Bright Land, Taiki manages to convince Yuu that DarkKnightmon is using him and tricked him into hurting others. However, SkullKnightmon abducts Yuu and spirits him off to Bagramon.(Source: Wikipedia)