Riku was an 18 years old Imanity who lived during the Great War on Disboard. He was one of the main protagonists in Tet's story. He encountered Shuvi while going on an expedition and brought her to his colony after having lost in a game against her (albeit willingly).

He bears many similarities with Sora, being his black hair, slender body, black eyes and his face. Riku is a master strategist and manipulator, much like Sora. He is capable of duping even the matriarch of the Nilvalen family (known as the strongest and smartest Elf at the time) into giving out valuable information, albeit at the cost of his health.

Ever since his home was destroyed and he took over as leader of his colony, he's had a somewhat split personality. When the figurative lock on his heart is locked, he appears emotionless, cold, and calculating. When his heart is unlocked, the emotions he bottled up are released (often violently). His endeavors with Shuvi has since lessened his split personality and he appears to be more open around her. He eventually marries Shuvi.
Through his efforts and skillful leadership, the Imanity were able to manipulate all the races into forming alliances with each other and drastically change the tide of the Great War, all while standing in the sidelines. It is implied that Riku is the first ever living being on the world of Disboard to create the concept of "cheating without getting caught." He is also the one responsible for the allocation of the 16 Race Pieces.
He led 177 fellow Imanity during the Great War against the Phantoms, which eventually caused the war to end, but with the sacrifice of both of his hands, left eye, all his skin being burnt, and organs damaged which led to his death.
(Source: No Game No Life Wikia)