Gen'yūmaru

was a prisoner of Orochimaru.

Abilities
While the only fight Gen'yūmaru has been observed in was blurred, the fact that he was chosen as Orochimaru's host speaks highly of his potential abilities. Moreover, he is observed taking his opponent's head off, and sending it flying accross the room, despite lacking any weapons

Sasuke Retrieval arc
When Orochimaru's body was damaged beyond repair, Kabuto Yakushi ordered Gen'yūmaru, 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'yūmaru a frightening illusion of a giant snake about to devour him, and Gen'yūmaru discovers that his body will become Orochimaru's next vessel. Knowing there is nothing he can do to escape this fate, Gen'yūmaru used his wish that Orochimaru offered him to request that his clan members that are also imprisoned be set free. With that, Gen'yūmaru's body was taken over.

Sai and Sasuke arc
After three years, Gen'yūmaru's body begins to reject Orochimaru and he attempts to take Sasuke Uchiha's body. When Sasuke attacked a greatly weakened Orochimaru, the latter abandoned Gen'yūmaru's body.

Hunt for Itachi arc
Gen'yūmaru 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'yūmaru, as she is informed that he was selected as the host body when she was too late to receive the same "privilege."
 * Gen'yūmaru is seen during Orochimaru's battle with Team Kakashi, when Orochimaru's face is struck and his face change jutsu is temporarily disrupted.