Oni (鬼) are creatures from Japanese folklore, variously translated as demons, devils, ogres or trolls. They are popular characters in Japanese art, literature and theater. Oni are known for attacking villages and plundering food, riches and women.
Oni is fought in a room where the protagonist finds a Magic Box which gives the protagonist an item needed to fight Yaksa.
Shin Megami Tensei III: Nocturne
"A common demon in Japanese mythology. A powerful red creature with a horn on its head that attacks villages with its iron club and plunders food, riches and women."
—Shin Megami Tensei III: Nocturne compendium
Oni are the main force of the Mantra Army, as several are seen populating both Ikebukuro and the Mantra Headquarters.
Aside from the regular Oni, there were also four Oni with special powers, Ongyo-Ki, Fuu-Ki, Sui-Ki and Kin-Ki, who were once the Mantra's elite, before somehow falling out with the army's commander, Thor. They were cast out of the Mantra Army and were left to become bandits in the tunnels leading to Asakusa.
Shin Megami Tensei IMAGINE
"Monstrous creatures of Japanese mythology. Some of them, such as Shuten-doji, are well-known throughout Japan, and it is common for "-doji" to be added to the names of famous Oni. Their entire bodies are built as tough as iron, they have horns on their heads and sharp fangs protrude from their mouths. Their thick, sharp claws can easily tear through tempered steel. They wear lionclothes of tiger skin and carry metal clubs studded with spikes. The number of horns and the color of their skin varies from Oni to Oni. The appearance of the Oni was established during the Heian period (794-1185). Because the so-called Oni Gate faces northeast, between the directions of the ox (NNE) and the tiger (ENE), the Oni was conceived as having the horns of a ox and the fangs and claws of a tiger. The animals who protected the Japanese folk hero Momotaro from the Oni were derived from the opposing directions: the monkey (WSW), the cock (W), and the dog (WNW). The prison guards of Hell who command the Oni are known as Mezuki and Gozuki."
—Shin Megami Tensei IMAGINE compendium
Oni can be contracted in Ueno and Ikebukuro during the in-game daytime hours. It appears as the final boss of Act 7 following a player's DB License test. He appears as an enemy in Ikebukuro as a demon contracted by punks, as a summon by the four oni bosses in Shinjuku, one of the invading forces in Shinagawa during the weekend invasion periods and the boss of the Suginami camp hacked chip instance. A non-playable Oni plays a central role in some limited events, such as Setsubun-themed events, summer beach events and a Valentine's event where he shows some interest in Moh Shuuvu.
Shin Megami Tensei: Strange Journey
"An evil monster of Japanese lore known for its hideous visage and brute strength. They plunder and loot villages, massacring the townspeople with their iron clubs. They have a horn on their heads, red skin, and razor-sharp teeth."
—Shin Megami Tensei: Strange Journey compendium
Oni appear as regular enemies in the basements of Sector Antlia. Several special Oni serve as Morax's special elite troopers. Another Oni appears as part of an EX Mission, Fool Eat Meat!, having indeed eaten Cerberus' special dinner.
Shin Megami Tensei IV
Oni can be found in Ueno. He can teach Flynn the Critical Wave, Taunt and Fang Breaker skills through his Demon Whisper.
Oni also makes an appearance in the "Training Battle 3" Challenge Quest alongside Dantalian. Using Blight on the duo will poison them, but this must be done before they have a chance to use Tetrakarn. Swapping in Zhen to use Pandemic Bomb, will also lower the pair's damage output.
Shin Megami Tensei IV DEMONIC GENE
Oni, alongside Fuu-Ki, is one of the first demons to attack Walter when he arrives in Tokyo. The Black Samurai is able to defeat both before they could finish Walter and a Casualry.
Shin Megami Tensei IV Apocalypse
Oni appears in all areas of Ueno. He will be accompanied by Leanan Sidhe in Ueno's initial areas and later appear alongside Sandman in the Shinobazu Pond area. He can pass along the Head Crush and Critical Wave skills during their whisper event.
Two Oni are also featured killing humans in the downloadable content Tokyo Destruction Promo.
Devil Summoner 2: Raidou Kuzunoha vs. King Abaddon
"An evil monster of Japanese lore known for its hideous appearance and brute strength. They loot and plunder villages, and massacre the townspeople with their iron clubs."
Oni is needed to perform a cross fusion along with Fuu-Ki, Sui-Ki and Kin-Ki, in order to create the ultimate persona of the Hermit Arcana, Ongyo-Ki.
"An evil monster from Japanese lore known for its hideous appearance and brute strength. They loot and plunder villages, massacring the townspeople with their iron clubs."
—Persona 5 background
Oni is the third Persona of the Strength Arcana and can be found as a Shadow in Kaneshiro's Palace and in the Kaitul area of Mementos, with the title "Chivalrous Fiend." It is one of four Personas to learn the Snap skill. When itemized through Electric Chair execution in the Velvet Room, Oni yields a Rampage skill card.
DemiKids Light & Dark
"Though fearsome to behold, this powerful demon is actually very kind and sensitive."
"And evil monster of Japanese lore known to plunder and massacre villages with their massive iron clubs. They have horns on their heads, long claws, and razor-sharp teeth. They are usually a red or blue color appearing in countless tales."