Also called En-no-Gyoja (or En-no-ozunu), he was born in Yamato no Kuni (present Nara prefecture) in the era of the emperor Jomei. He was wise in his childhood, pursued for classical books, through knowledge of the doctrine and mastered Sanpou (the law of Buddha, the laws, the priest). Later, he practiced ascetic training in Mt. Ikoma in Yamato (present Nara) and, in Mt. Kumano in Kii (present Wakayama prefecture), he climbed Mt. Katsuragi and enshrined Kujaku Myoo and practiced ascetic training, which gave him various supernatural powers, the Shugen, or Siddhi in Sanskrit. He also worshiped Kuzuryu, as a deity protecting Buddhism, and Kuni-no-Tokotachi, who has most of his shrines associated with the Shugendō religion. He was accused of entirely contrary to the fact, he was sent to Izu. He came back to Kyoto after being released and became a founder of Shugen-do (mountaineering asceticism, literally the Path of Siddhi), whose practitioners were treated with awe and superstition by the average medieval Japanese population. Later he was manifested in Buzen Kyushuu (present Ooita). He could not be traced then after. His servants are called Zenki and Goki, two yōkai he controlled, in a way reminiscent of King Solomon. He was deigned the title of Shinpen (or Jinben) Daibosatsu in 1799. En the ascetic appears quite often in Japanese fiction and pop culture as a cameo, usually to give an easy explanation to a character supernatural powers (Sadako, Katō Yasunori, etc.).
After Thor barrages Japan with ICBMs, the Heroine sends the protagonist to the Diamond Realm where he meets En no Ozuno. He asks the player to fetch him some Soma twice, and then locate Blue Phantom and Red Phantom (they will fight if the protagonist's alignment does not match either of theirs) to gain alignment-exclusive weapons (the Light Sword for Law and the Headsman Ax for Chaos). Afterwards, he sends them back to Tokyo 30 years later, but not before fighting his servants, Zenki and Goki.
En no Ozuno is encountered at the bottom of Twisted Tokyo after purchasing the "Depths of Twisted Tokyo" DLC ($1.49) and using the "Eternal Torus" item acquired from it. He is encountered on B57F and drops the Monastic Rosary as his unique equipment, along with Incense, Armor Boost Kits, Compendium Tickets, and Battle Plans. His unique equipment is the Neophyte's Rosary, an accessory that gives 273 MP and has the effect of High Pleromas on all of Nanashi's non-Almighty attacks.
En no Ozuno has a surprising weakness to physical attacks, allowing the player to potentially get Smirks and free Press Turns at no cost. However, this doesn't mean En no Ozuno should be taken lightly, as his attacks hit very hard. Zenki's Axe pierce resistances, but his other spells don't. Immunity to fire and electric attacks can rob him of his turns, but at times he may spend a turn using a Will spell to bypass this. He also occasionally throws up Tetrakarn to cover his phys weakness, but it can be easily bypassed with Phys Pierce, easily obtained off Matador who's become available in the earlier floors.
En no Ozuno has a couple of tricky moves that can waste the player's precious 10 turns they have while fighting him. Peacock Incantation causes En no Ozuno to counter the next attack with the same amount of almighty damage to the entire party. Goki's Water Wall completely maximizes his defense and agility boosts. There are ways to bypass this, though. Simply hitting En no Ozuno with a weak attack - even your basic physical attack - will force him to waste Peacock Incantation and do minimal amounts of damage to the party. Goki's Water Wall is only cast after En no Ozuno takes a lot of damage while having a defense or agility debuff, and can be avoided by never debuffing those stats. En no Ozuno does not have any form of Dekunda or Silent Prayer, and will do nothing to remove a -3 offense debuff. Spamming Tarunda or Venomous Raga makes the difference between surviving and immediate death. Doping is also recommended as it can't be erased by Dekaja which he uses every 3 turns or so. Also, should he be smirking and uses the incantation, both can be canceled via Magaon.
En no Ozuno also appears in the DLC, "Messiahs in the Diamond Realm." With his companions, Zenki and Goki, he instructs Nanashi to speak to Stephen to understand what is needed to do. Goki provides healing, while Zenki allows Nanashi to cancel the quest and return to his world. En no Ozuno serves as the rallying point for three of the Messiahs, before unlocking the door to the final fight of the quest.
In the Japanese version of Shin Megami Tensei IV Apocalypse, unlike the Fiends before him who spoke in dialogues completely consisting of kanji and katakana, En No Ozuno speaks normally, with his dialogues consisting of the standard kanji and hiragana.