In Colorful Moon Tsukihime, Oni are said to exist in the following varieties:
- Those who were from the start referred to as Oni, and fundamentally distinct from humans.
- Those humans who by way of aberration (変異, hen'i?) become as Oni.
The following is a list of known Oni that have appeared within the canon:
- Akuji no Takamaru
- Kazakoshimaru (風越丸, ?)
- Suzuka Gozen
- Todorokikemaru (轟カ丸, ?)
- Wazakuimaru (轟カ丸, ?)
In the past of Japan, many families wanted to crossbreed with these creatures. This resulted in the present clans of half-breeds, such as the Tohno and the Kishima. The relationship between the half-breeds and the oni is not unlike that between the Dead Apostles and the True Ancestors. Both the oni and the half-breeds have been in conflict with the Demon Hunter Organization for quite some time.
Those with thick blood are often overwhelmed by their blood with Inversion Impulses, and those that maximize the blood are called Crimson Red Vermillion (紅赤朱, Kurenai Seki Shu?). Also known as Ancestry Return, they are enveloped by an illusion-like fog and their demon blood overwhelms and consumes their human blood. Those that become Crimson Red Vermillion are never able to return to humanity again. Having a thick demon blood, Akiha Tohno has a high chance of becoming one. When her hair turns red, she is about to become one, but she's barely controlling herself and keeping a balance between the human's blood and demon's blood. The only other known Crimson Red Vermillion is Kouma.
 FATE - Day 10
――― the Phantasmal Species (幻想種, gensou-shu?).
As the wording implies, the term refers to those existences that persist (生存する, seizon-suru?, lit. "survive") only within Fantasy (幻想, gensou?).
Those Demi-Humans (亜人, ajin?) so called as Faeries (妖精, yousei?) or Giants (巨人, kyojin?);
those Demonic Beasts (魔獣, majyuu?) so called as Oni (鬼, Oni?) or Dragons (竜, Ryuu?).
They whose mode of being (在り方, arikata?) is inherently a "Mystery" (神秘, shinpi?) are merely in the act of "existing" manifest as existences that surpass Magecraft.
There exists a Principle (理, kotowari?) that a Mystery is extinguished in the face of a Mystery of greater magnitude.
So as Magecraft permits the accumulation of power in the mode of knowledge,
the Phantasmal Species accrue strength by the very measure of their lifespan (寿命, jyumyou?).
Taken to its limits by the assistance of Magecraft, the human corpus can persist for no more than five hundred years.
To they who have survived since distant antedeluvia (太古, taiko?), a Mystery on the order of half a millennium in magnitude is unworthy as competition.
However, the circumstance wherein Humanity and the Phantasmal Species inhabit same World is a matter of the past.
Those of the Phantasmal Species that are long-lived grow to some extent distant from this World.
At present, the Phantasmal Species that as yet remain within the World are those existences whose standing may be expressed in units of a century.
 Tsukihime Dokuhon PlusPeriod - TSUKIHIME Dictionary: Crimson Red Vermilion [Term], p.178
Crimson Red Vermilion [Term]
The name for when those with inhuman heritage have drawn out their sleeping blood to the maximum possible extent. Once they reach this state, they are wreathed in a mirage-like haze and can never return to being human.
The name Crimson Red Vermilion is unique to the families ruled over by the Tohno household. Elsewhere, it is known as Ancestral Return.
As one with particularly tainted blood, the possibility that Akiha will become a Crimson Red Vermilion is high.
In Tsukihime, when her hair turned deep crimson, she was on the verge of this, though somehow she managed to bring her human and oni blood under control. Of course, rather than returning to how she was before, it's more accurate to say she switched to her "human" self while remaining close to the state Crimson Red Vermilion.
 Melty Blood Act Cadenza PS2 Manual - Dictionary: Crimson Red Vermillion [Term], p.057
Crimson Red Vermillion [Term]
Crimson Red Vermillion.
The name for when those with inhuman heritage have drawn out their sleeping blood to the maximum possible extent.
The name Crimson Red Vermillion is unique to the families ruled over by the Tohno household. Elsewhere, it is known as Ancestral Return.
As one with particularly tainted blood, the possibility that Akiha will become a Crimson Red Vermillion is high.
You can catch a glimpse of this during Tohno Akiha and Kishima Kouma's Arc Drives. Also, though Kouma is the closest to attaining it at the current stage, Akiha is actually the one with more aptitude for it.