Universe 15 Oni, the Broad Strokes
Anatomy/Physiology
The matter of Oni anatomy is a strange thing in and of itself. While it is true there are baseline characteristics (to be noted below), there are several key differences that set the Oni as a species apart from most others, and even themselves. So we'll keep it simple ... All Oni, be they old, or young, possess one several things that do not vary regardless of how they are born, or made (more on this below). All Oni possess horns, and the same internal organs, musculature, and bone density. These organs are largely similar to many of the bipedal humanoid species (heart, lung, kidneys, etc) yet an Oni's pulmonary and respiratory systems operate at a much higher efficiency. An Oni's bones, too, are naturally denser than that of other species, and while the reason for this is unknown, this generally means that an Oni of the same height and body type of a Human, or Saiyan, generally weighs 25kg more on average. Oni are also- owing to their place of residence- permeated with innate magical energy. This means that, while most are unable to utilize magic- simply for never learning- they all possess the knack. In addition to this, while none are quite sure why ... Oni are also possessed of telepathy. Limited in scope to barely be capable of much more than communication without the need for verbalization; there are some Oni in times passed who have been stated to be capable of more. A thing that itself should be taken quite seriously as, another innate trait of all Oni of Universe 15, is that they are fully incapable of lying. An attempt is simply met with their inability to actually speak, or think, the intended lie. As a result, Oni are quite adept at weaving the truth, being able to navigate around something they do not wish to disclose without actually giving away the 'truth'. A loophole, perhaps, but a thing that 'works'.
A few final things of note about the physical similarities of Oni, are that of the horns, and skin; of which both are as numerously varied as there are stars int he sky. There are a few guidelines however, in regards to this. While an Oni can be born with any color of skin that can be imagined ... they are usually hues of blue, and red, and the horns are typically colored to match. In regards to the horns, however, their length, thickness, and number all vary greatly. Composed of bone, they exist as outcroppings that protrude from the top of the forehead, and simply serve as a natural means of protection, and defense; and while the number of horns usually never exceeds three, there is a legend told among the younger Oni of a legendary Oni possessed of ten horns.
This is of course ludicrous however, as the Oni they speak of with giggles and whispers, only possessed five. Besides that, he is not even dead. He simply lives a meager existence at the Edge of the Timeless Garden nowadays, referred to by most who know him as the "Quinotaur".
... A thing, he assures you, he is not at all fond of.
Functional Immortality/Children
A thing largely assumed to be the result of their nigh-perfect physiology coupled with their innate magical nature, and the consumption of the magical flora of the Gardens they generally reside in; barring mortal injury and/or illness, an Oni will exist for as long as the Timeless Expanse of Otherworld. To that end, Oni children are extremely rare, and a thing that garners much attention by all Oni. After all, while Oni are capable of procreation, and do engage in the act after careful consideration and choosing a willing partner; most of the Oni residing in Otherworld were created ... made. Now, possessed of the same anatomical and physiological traits and similarities a 'made' Oni, and an Oni naturally born are all but impossible to tell apart; the difference comes when the matter of their childhood is discussed. A 'made' Oni, was never afforded the luxury of childhood. Created as they were with a specific purpose in mind, there was simply no need, and so they are simply created at the end of the 'development phase'. Oni Children however, born from a pairing however- while as noted is extremely rare- are allowed to age to the end of the 'development phase' naturally. This 'phase' generally lasts twenty to thirty years and is similar to all species who rear children.
Naming Conventions
For the 'formative years' after their awakening (or birth), Oni are simply named after the color of their skin and any other stand-out traits unique to their physiology. In this, they are similar to the Oni of the multiverse, who in general choose names of a similar bent (Blue Two-Horn, Red One-Horn). This is where similarities end however, as, after determining their assigned task, be it protection of the Timeless Expanse, the shepherding of the souls who pass through, or tending to the Timeless Gardens; they are assigned- or more commonly are allowed to choose- a name based off of the ever-growing list of deities of fallen civilizations that pass through the gateway into Otherworld. While this does cause some confusion, especially when multiple Oni who are assigned the same task pick the same name, the addition of an epithet ("Green-Eyed Aphrodite", or "Long-Horned Hermes") is usually afforded to mitigate this. So while there may be several Oni named 'Vishnu', there is only one "Three-Horned" Vishnu, and so on. There are also exceptions to this rule, though they are few and far between. With the most prominent exception being a Mrs. Landinghorn. A wonderful lady, though only Zeno-Sama knows why her horns are landing.
Culture/Language
A species bred/made for an endless work, Oni culture, and language is a strange thing. A largely peaceable species, tasked as they are with gardening and keeping the peace while shepherding the countless dead, they have no 'set' of traditions, or doctrines, or even overarching beliefs. They simply incorporate aspects from other cultures that they themselves find appealing. Thanks to their lifespans, this means that the aspects that are usually incorporated are typically old and long-forgotten ... much like the names they take for themselves. Given the size of Otherworld and sheer number of the souls who pass through, however, there are quite a few groups, or 'tribes' of Oni that entertain various aspects of the civilizations from the countless souls who pass through; the largest of these 'tribes' however are centered around the cultures of Ancient Japan, and Mesopotamia. To that end, Oni language- and History- is similarly strange. When left to their own devices, an Oni will simply rely on their innate telepathy to converse with others, not feeling the need to speak with someone in person when they can simply use their mind to speak with them thousands of kilometers away. When dealing with others however, they eschew this practice in favor of direct conversation preferring to use the Universal Common Language, 'Common', or assuming they are afforded an opportunity; learning the language of those groups of souls they deal with most. There is no written language however, owing to the fact that their functional immortality renders the need to record things somewhat moot as there is usually a dozen or so more Oni who were 'alive' for the moments or things that would require written recording. It is a general practice then, when an Oni gets curious (more on this in 'Other'), to simply seek out an Oni who was around at the time to ask them. This also means that, in general, Oni do not fully grasp the concept of Death in context to themselves since most of them are alive and well. Except for the Progenitors, and we don't talk about them.
Other
There is a thing to be said (perhaps many things), about an Oni's curious nature. Given their secluded lives, and the fact that they are constantly subject to new things thanks to the billions of souls who pass over, Oni are naturally given to their curiosity. That they are similarly unable to lie, or process deception, means that they are far and away more naive, and innocent in regards to their curiosity than other species. As a result ... there is a more than insignificance chance an Oni will do something in an attempt to sate their curiosity without fully understanding the implications, or context. And so long as the Oni remains in Otherworld this generally leads nowhere dangerous. The issues generally occur however, when an Oni is afforded the opportunity to leave, be it through discovering a gap by which to slip from, becoming exiled from Otherworld- for crimes such as murdering Kin- or simply being offered an out by powerful entity. Once freed from their servitude and sheltered life as caretakers of Otherworld, the freed Oni generally lose themselves to curiosity and their own desires; generally earning themselves the ire of the 'waking worlds' due to their lack of understanding of societal values ... in essence, they simply take, break, and otherwise do what they want.
And while they remain largely benign in their curiosities- their social faux pas being nothing more than 'being naked all the time', there are accounts of several Oni who have fallen to darkness entirely, those few seeking out dark magics and power enough to become the very Gods they served; or simply deciding that they enjoy the act of killing, and having power enough to decide the fate of another. This backfires as often as it doesn't however, with many of the Oni's who have made these Faustian Bargains finding themselves rallied against and imprisoned; or in extreme cases dealt with by the very Gods they sought to challenge.