Why I made this post.
Devils Devel Concept is one of my favorite VNs and has one of my top 3 VN settings. The setting is so ridiculously complex and explained only in fragments along the way, so it is difficult to gain any real grasp on what is going on just reading any one path... Not to mention that there are a ridiculous number of details that can be lost along the way as you read. Sora is fundamentally an indifferent narrator when it comes to such details, because he doesn't have any interest in them, and the heroines aren't much better that way, only dropping tidbits along the way that can be easily misinterpreted without being able to see the whole picture. There are unavoidable spoilers in this post, but they aren't so much for the story as for the infrequent explanations of the setting the writer inserts along the way. In particular, I recommend skipping the part of the post covering the Genryuu, Old Ones, and Others and their servants until you've at least read both Kanata's and Mutsuki's paths. Mutsuki's path, in particular, takes on a lot of new meaning if you know the stuff about the contracts. This is mostly a glossary, drawing some base info from the in-game glossary but also adding parts drawn from the explanations within the story itself.
Possible Spoilers if you go any further
Demons- Powerful beings capable of rewriting reality at will. They are fundamentally inhuman in both appearance and motivation. They are the descendants of the creations of the Old Ones, possessing Devils Organs passed down directly from that source.
They are separated into clans by which Old One was responsible for the creation of their ancestors. Demons can live for centuries, with the 'purest' of them sometimes living for thousands of years. Demon society has lost most of its knowledge of its past, as the original generation after the Old Ones is long-since dead. Demon procreation is rare at best, and so most Demons are 'recycled', the souls of the dead retained by the Sage of their Clan until it can be implanted into a new body. The difference in power between Clan Monarchs and Sages vs the average demon is equivalent to a master swordsman vs a peasant armed with a toothpick.
Colors- The Clans of the Demons, each led by a monarch who rules/organizes them, and possessing a Sage that serves as a repository of souls and Organs to be passed to a new generation. The clans are: Blue, Red, Gold, Black, and Green, based off of the five Old Ones from whom they are descended.
Enja- The descendants of humans implanted with Devils Organs and an instinctive need to kill Demons by the Old Ones upon their defeat. They breed slowly amongst themselves and are extremely short-lived (with rare exceptions for those who rarely use their abilities, most don't live beyond the age of forty, with the most powerful tending to die or Fall in their twenties). The imperatives in their DNA have a lot of subtle psychological effects on them, ranging from mild social disorders to outright sociopath behavior. Their desires and hungers tend to be several degrees stronger than the average human, as well as being somewhat more... earthy. Out of self-protection, they tend to gather in clan groups based on bloodline or association.
Hunters/Karyuudo- Enja who have chosen to pursue a life hunting and killing Demons.
Henshitsu- As an Enja wields their Organs and live their lives, gradually they begin to alter, in fashions both subtle and not so much so. This can be displayed in strengthened instincts, vulnerabilities to certain types of food or light, the need to drink blood, or even the ability to live on sunlight and water. Once this change surpasses a certain point, an Enja falls and becomes a Stray Sheep, a monster without rationality or conscience that is driven by the most powerful impulses of the Enja from which they were born.
Stray Sheep/Predator/Hosokusha/Itsudatsusha- Individuals whose Henshitsu has gone beyond the critical stage. Such individuals lack rationality or conscience and are often cannibalistic. They are also often driven by strong suppressed desires or obsessions from their life as a human. Sometimes they are born when an Enja dies in a conventional manner (of age, starvation, etc), but most of the time it tends to occur during battle or another extreme situation.
Ro- Unique Devils Organs, usually based off a concept or element, that consume the concept or element they were named for and grant power to their possessors.
A possessor of a Ro is one of the two most powerful Enja types in existence. All Ro users are female. Ro users are the second most psychologically broken types of Enja (Genryuu being the most broken) and share very little with normal humanity beyond a few basic emotions. Ro users tend to be impulsive, selfish, amoral, and extremely yandere. Enja Ro are extremely short-lived, as their intense desires combined with the uncontrollable need to use their power inevitably cause them to self-destruct. It is generally expected that the average Ro user won't survive to hit their thirties, and their fertility rate is extremely low. As such, it isn't uncommon for them to be encouraged to find sexual partners early and produce children as young as possible. All Demon Sages and female Monarchs possess a Ro.
Genryuu- A type of Enja, created by the Others rather than the Old Ones.
They, unlike their fellow Enja, are bound by a strict contract determined at the origin of their bloodline, and if they fail to fulfill its terms, they become 'normal' Enja, losing much of their power. Genryuu do not suffer from Henshitsu, nor do they fall and become Stray Sheep. All Genryuu are marked by a peculiar organ 'gifted' to them by the Others known as 'Hakai no Mugen Bunki', or 'the Infinite Branching of Broken Commandments'. This ability forces on them 'dreams' (waking or sleeping) of all potential futures, pasts, and presents, giving them immense knowledge that potentially allows them to subconsciously alter the vector of fate. As a result, Genryuu are generally considered to be the most powerful of all Enja by several degrees. Genryuu personalities are shaped to some extent by the necessity to survive possessing Mugen Bunki, and they tend to be defined by a tendency to live in the present, be indifferent to most matters, and to be almost mechanically driven to seek out and destroy demons.
Furuki Mono- The Old Ones, otherworldly beings who survived the void to arrive on Earth long ago.
They destroyed their own homeworld due to their aggressive, combative instincts, and even on this new world, the 'originals' found themselves unable to control these impulses. In an attempt to weaken these impulses, they created 'children' based off of a mix of the native intelligent species and themselves (the Demons), but this merely resulted in them ultimately being killed by their own children, their corpses dismembered, and the remains sealed away in Mausoleums all over the world. They created the Enja as a final revenge against their descendants.
The Others- Otherworldly beings who preceded the arrival of the Old Ones.
All lifeforms on Earth are descended from them to some degree. Out of revenge, they created the Genryuu bloodlines, and those few that survive have done so by living on a different plane of existence, from which direct intervention is difficult, if not impossible.
Servants/slaves sent to watch over Genryuu by the Other with whom the said Genryuu is contracted. This seems to apply primarily to the initial contractor rather than later contractors within the bloodline, but there are some hints that such spirits follow around the descendants as well. They are invisible to even Demons, save those with special 'eye' Organs, and they seem to primarily serve as a spiritual mirror to the Genryuu most of the time. They have no combat abilities and are rarely physically present in the mortal realm.
Shinigami- Sora's half-insulting name for the Other with whom he is contracted. A cruel, cold, faceless feature who seems to be female (uncertain).
Solid pot- A discriminatory term used by Enja to refer to themselves.
Hollow Pot- A discriminatory term used by some Enja to refer to normal humans.
Soujou-Soukoku- A reference to the natural magnetism/compatibility between individual Enja and Demons. This is the first and most powerful determiner of how a relationship between them will form, and it isn't uncommon for those with a high compatibility to fall into bed on first meeting or those with a low or negative compatibility to try to kill one another at first sight. Individuals with high compatibility enhance one another's abilities, whereas those with low or negative compatibility make one another weaker over time.
Masou- Weapons created by the Enja to allow them to more effectively fight the Demons. These enhance their users' abilities (though usually at a cost in lifespan or psychological damage) and can provide new methods to corrode reality. Many have fallen into the hands of the Demons or have been partially broken over the millennia, though.
Shinshoku- A term referring to the ability to corrode reality that defines both the Demons and Enja. The only limits to this ability are the individual's Organs, the amount of power they can produce at a given time, and the size of the area they can comprehend with their spatial awareness.
Battle Flow- Battles between possessors of Devils Organs are a competition to see who can overwhelm the other's ability to corrode and alter reality. It isn't uncommon for Enja and Demons to die multiple times during a battle, resurrecting themselves as they go, and fatal blows generally are merely minor turning points in the battles of the most powerful. To achieve victory, it is necessary that one 'rewrite' the opponent into a 'deceased' state at a point where the balance of power in the battle has tipped irrevocably against the opponent. At lower levels, one or two deaths is often sufficient to end the battle, as resurrection and bodily repair require an iron will and a decent grasp of one's own physiology before it was rewritten or destroyed. More intelligent opponents prefer to create a situation where their enemy has no possibility to turn the tables in advance by choosing the field and 'corroding' its reality in advance (creating a branch world for that purpose). Kanata, Misora, and Mei are all three of this type, whereas Sora, Mutsuki, and Akane tend to be straightforward 'smash the wall down' types.
Relationship between Demons and Enja- Demons and Enja instinctively hate one another on sight. This instinct is literally beyond their ability to control or suppress, and it is only through the mediation of a third party that the few 'rules' of the conflict between them were put together. The reasons for this instinctive hatred are generally considered to lie in the nature of the Enja's creation.
The Old Ones (Furuki Mono), when they were being killed and sealed away by their own descendants, the Demons (with the help of an unnamed third party), implanted seeds of their own nature inside of humans, who then evolved into the Enja (those that didn't die or turn into Stray Sheep). The Devils Organs given to the Enja fall into two categories... those 'stolen' from slain demons and those that come directly through descent from the Old Ones. Demons consider Enja to be thieves and blasphemers, as Demon society has warped badly in the time since the Old Ones passed, causing them to lose full knowledge of their origins and the reasons why the Old Ones died. Enja despise Demons because that was inserted into their ancestors along with the Devils Organs, imprinting an unstoppable impulse to kill Demons into their DNA.
Blood (Demon and Enja)- The blood of Demons is the most poisonous substance in the world. A human touched by a drop will die instantly, and even Enja often become ill from excessive exposure. To a lesser extent, the blood of Enja is toxic to Demons, but it takes many hundreds of exposures for this to actually become a problem for the Demon in question.
Devils Organs- 'Organs' that grant Enja and Demons the ability to rewrite reality, adhering to rules determined by the individual Organ. Most Enja only possess one or two, with a few rare exceptions being born with three (generally destined to have a very short lifespan). While all Enja and demons can corrode reality, in what manner they do so is determined by these organs.
Cabals- Organizations devoted to the protection, training, and organizing of activities by the Hunters. The ones most intimately involved in this story are the Isaribi and the Houjou. Both have a very pre-movie MIB feeling to both their activities and their attitude, as their agents tend to spend as much time disposing of evidence and witnesses as they do actually fighting Demons or hunting Stray Sheep.
Dolls- Humans rewritten into 'modoki/automata', flesh machines that have been granted immense physical strength, agility, and speed to serve as the hands and feet of a Demon, Stray Sheep, or Enja. These individuals rarely retain much of their original personality or memories, and most are cannibalistic, needing to regularly eat large amounts of human flesh to maintain themselves. Ironically, even the least of the Enja can dismember them with hardly any effort, making them no better than a distraction in the battle between the two.
Fear in Enja vs Fear in Normal Humans- Both humans and Enja are capable of feeling fear towards Demons. However, when a human encounters a Demon, they are incapable of even conceiving of resistance or putting up a struggle. When an Enja encounters a Demon, their fear is a simple product of a mixture of threat assessment and jealousy of their power. Fear of Demons freezes a human, whereas an Enja is still capable of acting. This peculiar type of half-rational emotion is one of the things that set Enja apart.