High School DxD: The Infinite Dragon, The Black Dragon Emperor

Kazuto Gremory was the son of Zeoticus Gremory and Venelana Gremory as well as the younger brother of Sirzechs and older brother of Rias. Everyone liked him until one day something happened. Riser, son of the Phenex family tried to kill him by sending him through a portal and he had his chance to claim his sister, since that day, Rias went into depression due to his brother's loss.