Gen'yūmaru

was a prisoner of Orochimaru.

Sasuke Retrieval Arc
When Orochimaru's body was damaged beyond repair, Kabuto ordered Gen'yumaru, along with many other prisoners, to fight to the death, with the understanding that the survivor will be set free. When he wins, Orochimaru's entrance to the arena gave Gen'yumaru a frightening illusion of a giant snake about to devour him, and Gen'yumaru discovers that his body will become Orochimaru's next vessel. Knowing there is nothing he can do to escape this fate, Gen'yumaru used his wish that Orochimaru offered him to request that his clan members that are also imprisoned be set free. With that, Gen'yumaru's body was taken over.

Part II
After three years Gen'yumaru's body begins to reject Orochimaru and he attempts to take Sasuke Uchiha's body. Gen'yumaru is later seen in Orochimaru's subconscious along with his other former host bodies' souls.

Trivia

 * literally means "confined to an illusion". When written as 原由, it means "reason".
 * Guren is suggested to have known Gen'yumaru, as she is informed that he was selected as the host body when she was too late to receive the same "privilege".