Gen'yūmaru had tousled neck-length white hair, a strong chin, and dark purple eyes that changed to amber snake-like slits when Orochimaru took control over his body. He wore black wristbands with thick purple rope wrapped around his chest over a tan shirt, and black trousers.
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 who was 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 who are also imprisoned be set free. With that, Gen'yūmaru's body was taken over.
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.
Gen'yūmaru is later seen in Orochimaru's subconscious along with his other former host bodies' souls.
Gen'yūmaru's body is failing Orochimaru, causing him to be bedridden. In the hopes of extending his host's vitality before he could transfer, Orochimaru ordered Sasuke to bring a man named Shinnō to Otogakure, who knows many Reincarnation Ninjutsu that may help. In the end, Sasuke failed to bring Shinnō in person, but a scroll that contains the necessary information to extend Gen'yūmaru's lifespan.
He was described as a firm, and greatly self-confident individual.
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 across the room, despite lacking any weapons. However, Gen'yūmaru's body was not strong enough to last Orochimaru for more than three years, as it began to weaken and reject him.
- Gen'yū (幻幽) 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 by Naruto and Orochimaru's face skin is temporarily torn