Gen'yūmaru

was a one of Orochimaru's prisoners and his last body.

Sasuke Retrieval arc
He was told to have a fight to the death with the others that Orochimaru had kept captive, and the winner would be able to go free. Gen'yumaru was the sole survivor of them all, only to find out that he was going to be the next vessel for Orochimaru. As his last wish, he asked for his clan to be released, which was being held captive for Orochimaru's experiments. It is unknown if this request was ever granted. Orochimaru noted that part of Gen'yumaru's consciousness would remain within him after the transfer — making Gen'yumaru still alive, in a sense.

Three-Tails Arrival arc
It is implied that Gen'yumaru and Guren knew each other, since a flashback in Guren's memory reveals that she came to Orochimaru's hideout the day of Orochimaru's last reincarnation to be told by Kabuto that Gen'yumaru had been chosen as the new vessel, since she was too late to enter the fight with the other captives.

Orochimaru's Demise
After Orochimaru abandons Gen'yumaru's body, Gen'yumaru still remains within his subconscious.

Trivia

 * literally means "confined to an illusion". When written as 原由, it means "reason".