Gen'yūmaru

was a prisoner of Orochimaru.

Sasuke Retrieval Arc
He is forced to fight his fellow prisoners with the understanding that the survivor will be set free. When he wins he discovers that his body will become Orochimaru's next vessel. Knowing there is nothing he can do to escape this fate, Gen'yumaru asks that his clan members that are also imprisoned be set free.

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".