|Novel||Sasuke Shinden: Book of Sunrise|
Oyashiro En is a medical-nin and a kekkei genkai collector.
Majority of Oyashiro's past is unknown. He became a medical-nin but he developed a keen interest in people who possessed a kekkei genkai. His obsession led him to kidnap several shinobi from various villages and hold them captive so that he could earn money from rich people who wanted to hire such shinobi for missions. He would also stage fights between different shinobi with his clients in attendance to determine which was the strongest in an arena called "Coliseum". This earned Oyashiro to be condemned as a international criminal and he went into hiding in various hideouts, similar to Orochimaru, who admires him and has met him at some unknown point in time.
Oyashiro is a greedy and eccentric individual who cares more about collecting kekkei genkai owners in exchange for money. Oyashiro found it to be easy to kidnap small children, such as Chino, to keep up his habits compared to kidnapping adults. He would raise these children into growing up as violent and merciless shinobi to increase the powers of their abilities, as well as make them more impressive to his potential clients. His cruel treatment towards his victims caused him to become a target of hatred from them and they envisioned getting revenge on him, but he finds these feelings unworthy. Oyashiro seems to care less about his status as a criminal and he retorts at Sasuke for once being one when the latter criticised him for exploiting people.
Oyashiro's abilities are unknown. All that is known is that he is a strong medical ninja.
Sasuke and Orochimaru arrive at the Coliseum to meet with Oyashiro, whom Orochimaru says likely has information on the Dark Thunder Group and the original Lightning Group. Oyashiro is hosting a tournament, and upon seeing Sasuke's Sharingan, he abruptly names Sasuke as a participate with the hope of obtaining it, much to Sasuke's dismay. Oyashiro sends his best shinobi, Futsu, to fight Sasuke and Futsu is quickly defeated. Sasuke demands for the information he desires, and Oyashiro explains that Fushin (the leader of the Lighting Group) became his protégé after losing a match in the Coliseum, but ran away with all of Oyashiro's kekkei genkai owners. Sasuke berates Oyashiro for his behaviour, only for him to retort that Sasuke, who is collecting information for others, is no different. Angered, Sasuke wins the tournament and frees all the shinobi, shocking Oyashiro. Left with no choice, Oyashiro tells Sasuke the location of Fushin's birthplace, the Valley of Hell, which the organisation destroyed out of resentment.
- Oyashiro derives from the Japanese shrine Izumo Ōyashiro, which is known for matchmaking for business and personal success.