Devil

The Devils (悪魔 Akuma) are one of the Three Factions in High School DxD, alongside the Angels and Fallen Angels.

Appearance
Pure Devils look exactly like humans, albeit with at least one pair of black bat-like wings that can be hidden at will. However, those from the Phenex family do not have the usual bat wings, instead having eagle-like wings of fire. It is still unknown if Devils can have multiple wings like the Angels and Fallen Angels, though Vali, who is a Half-Devil/Half-Human, has shown Issei "multiple" pairs of bat wings in Volume 4 (with four pairs in the anime) and Rizevim Livan Lucifer has shown to have twelves bat wings in Volume 18. Reincarnated Devils can come in a variety of shapes and sizes, such as the case with Tannin, a former Dragon King, Koneko Toujou, a Nekomata, and Enku (a Qilin).

Female Devils tend to utilize magic in order to appear much younger than their physical age would suggest, which explains why Venelana Gremory looks as if she were the same age as her youngest child, Rias.

Abilities
All Devils have increased physical abilities, such as superhuman strength, endurance, with enhanced senses such as an increased hearing range and eyesight. In addition, as they are creatures of the night, Devils have the special ability to see clearly in the dark. Devils also have the ability of flight through their bat-like wings. All Devil possess a passive ability, called "Language", where people listening to them will hear it in the language they are most familiar with; and vice versa. According to Rias in Volume 4 of the light novels, Devils have an immunity towards sunlight.

High-Class Devils are able to gain peerages, rule over other Devils depending on their family's rank and powers, Can have up to 15 "servants", which are represented by each piece in a chess game (Rook, Knight, Pawn, etc.). The High-Class Devil takes the position of "King". Lower-Class Devils are often looked down upon, and usually serve a Devil of higher authority. High-Class Devils also have the ability to reincarnate Humans and other races into Devils through the Evil Pieces system, as seen with what Rias did with Issei and Asia Argento.

Just one of the Four Great Satans could destroy the nation of Japan several times over with their power. Whether this is true for all Ultimate-Class Devils or not is unknown, but it does give an indication of their power.

Members of High-Class clans appear to inherit special innate magical abilities unique to that clan, though occasionally descendants of those who marry out of the clan will also inherit these abilities. Like thoughts of the Phenex's Clan who have the power of immortality, the Bael's and Gremory Clan who has the Power of Destruction, and the Abaddon's Power of Hole. The origins of these powers are large unknown. The pure-blooded Devils who don't inherit such powers are looked down upon, even by members of their own family.

Devils also have the ability to cast magic spells, not limited to those in which their family specializes. Examples being the ability to bind and later summon a Familiar, as well as Issei's Dress Break and Bilingual spells. The most common usage would likely be the magic circle used by Devils for travel, and especially to make contracts.

As revealed in Volume 14, to use demonic powers, a Devil must first acquire the power of imagination and the power to create, as well as have a good sense.

Weaknesses
A common weakness for all Devils is that they cannot go near/get involved with anything that has to do with the Biblical God, as they will suffer nasty side effects (headaches, immense pain, etc.), as seen when Issei couldn't go near the Church when he was talking to Asia, or when he tried to pray but only ended up collapsing from the pain soon after. They are vulnerable to holy objects such as crucifixes, holy water, and the Bible, and can get killed easily by Holy Swords and Sacred Gears with divine properties, such as Blade Blacksmith and the True Longinus (Asia's Twilight Healing, however, is a notable exception). These effects are caused by the system put in place by the Biblical God, which is being kept up by Archangel Michael, and only he has the power to change the system.

They are also susceptible to light which significantly drains them of their energy once hit (in Volume 1, Issei comments that sunlight also appears to drain him of his energy, reducing him to normal human levels, at least at that time). Also, intense light, such as the light-based weapons used by Angels and Fallen Angels and the light swords used by exorcists, can severely wound, if not kill, a Devil. Devils with higher demonic power, however, can develop a certain degree of resistance to light.

Ranking
The Devils use a hierarchical system that is split into four different ranks. The ranking here refers to the hereditary title given to Devils base on their classes and clans and do not reflect on strength. They are: There are three different ways to raise one's ranking: Once a Low-Class Devil has earned the right to raise their ranking, they must first be tested before actually gaining the position. Not everyone succeeds in these tests, but they can try as many times as they choose, as the permission does not normally expire. The Middle-Class promotion test has three parts that are modeled after the tests in the Human world since a majority of the Devils who taking the promotion tests are reincarnated Devils (former humans). The first part is a written report that addresses the theme of aim and greed. The second part is written test involving the history and government of the Underworld. The third is a practical where an individual faces off against another participant from the test in a one-on-one fight. During this battle, Pawns are given special permission that allows them to promote as if they were in enemy territory. High-Class Devils and above are allowed to have their own "peerage" which consists of members who represent each piece in a game of Chess while for reincarnated Devils that have reached Ultimate-Class, they will receive the right to choose a part of the Underworld as their territory. There is actually no conflict when a Low-Class Devil starts his own group while still being a part of another peerage after becoming High-Class. When a Devil reaches High-Class, they receive a set of Evil Pieces to start their own group. However, even after going independent, Devils are still obliged to fight as underlings whenever their master has a Rating Game, for example, if Issei were to reach High-Class and start his own peerage, he would still have to fight as Rias' Pawn if she ever has a Rating Game.
 * Ultimate-Class Devils (最上級悪魔 Saijōkyū Akuma): This class is the second strongest that Devils can gain by training or be born with the power. As an devils becomes into the class they gain a rank in the underworld as Satans, Great King, King, Archduke, Archduchess, Duke, Duchess, Prince and Princess.
 * High-Class Devils (上級悪魔 Jōkyū Akuma): Is the Three class most of devils are mostly the younger devils who have yet to master their power. Includes Marquis, Marchioness, Margrave, Margravine, Count, Countess, Viscount, Viscountess, Baron, and Baroness.
 * Middle-Class Devils (中級悪魔 Chūkyū Akuma): Is the middle class of new devils who have passed one of the three raise ranking. Includes Baron, Baroness, Knight, and Dame
 * Low-Class Devils (下級悪魔 Kakyū Akuma): The weakest class who are usually reincarnated as a Devil or are just commoners. They are a part of a High Class Devil's peerage and serve one of the higher class. Low-Class Devils can promote themselves to higher rankings depending on their performance.
 * 1) Contracting: Doing Devil Jobs by granting the client's wish. This includes having a high number of contracts completed and/or receiving high quality rewards. Devil contracts will garner points over time, but it is a gradual process unless the human customer is someone with great power and influence (such as a president or leader of a country). This is considered to be the slowest method; also, a promotion through contracts is seen as more of a tradition.
 * 2) Military Accomplishments: Obtaining great achievements in times of crisis or situations. This includes showing exceptional ability or skill in combat (rare even before the Kuoh Treaty as everybody feared mutual annihilation if another war broke out). By defeating high ranked enemies of the Devils, such as leaders of the Angels or Fallen Angels, a Devil can be rewarded. This is the fastest method, however, this route is unlikely in the current stalemate (as of Volume 4) without a full-scale war.
 * 3)  The Rating Games: The last method is by doing well in Rating Games. This includes earning high points, public esteem, and opinion in official Rating Games. This is the modern path for ascent in Devil society, as it was designed to promote and train new powerful Devils. The Rating Game allows reborn Devils to hone and show off their skills and abilities against others in a controlled non-lethal environment. Performing well in the Rating Game raises reputation and leads to promotion. In fact, the Top 10 rankers in the Rating Game have all reached Ultimate-Class, with the 7th ranker being the highest ranked reincarnated Devil. This is by far the best method to raise one's own ranking.

Currently the known "Peerages" are as follows:
 * Rias Gremory's Peerage
 * Sona Sitri's Peerage
 * Riser Phenex's Peerage
 * Sairaorg Bael's Peerage
 * Sirzechs Lucifer's Peerage

Four Great Satans
The Four Great Satans (四大魔王, Yondai Maō) are the four rulers who control the Underworld. The current Four Satans are not the actual ones but were chosen to replace the four original Satans who died in the Great War, and each was named after the four original Satans: Lucifer, Beelzebub, Leviathan, and Asmodeus. Therefore the names "Lucifer" and "Beelzebub" are not personal names, but are more like managerial positions.

72 Pillars
The 72 Pillars (72柱, Nanajūni Hashira, referring to the 72 demons of the Ars Goetia) are a list of the 72 families of pure-blooded Devils (known as Pure Devils). Each clan possessing dozens of armies under them. After the Great War, however, every clan lost a majority of their armies and over half of clans from the 72 Pillars even faced extinction, with only 33 clans remaining (in reality, there are more than 33 houses that are still active but have lost some of their status in the 72 Pillars due to having mixed heritage).

Extra Demons
Extra Demons are the houses of top-class Devils who are not a part of the 72 Pillars. Currently most of the Extra Demons have secluded themselves from the current Devil Society, not wanting to involve themselves with the Government. Devils from the Extra Demons who involve themselves with the current Devil Government will have to break all ties with their households such as with the case of Kuisha Abaddon, Bedeze Abaddon (the 3rd Ranking Champion in Rating Game) and Roygun Belphegor (the 2nd Ranking Champion in Rating Games).

The only known houses are the House of Lucifuge (where Grayfia comes from), the House of Abaddon (where Kuisha comes from) and the House of Belphegor. Mephisto Pheles is the only Extra Demon that doesn't have a clan. A majority of the Extra Demons serve under one of original Satans, such as the House of Lucifuge serving the Satan, Lucifer.

Stray Devils
Stray Devils (はぐれ悪魔 Hagure Akuma) are Devils who have diverted away from their masters. Without their masters to keep their powers in check, they become a great threat if their powers go beyond their control. The Devils take this case very seriously, and any Devil who becomes a Stray will be killed on the spot. The only known Stray Devils in the series are Viser. Kuroka, the latter coming from the Vali Team and was the only known SS-Class Stray Devil as the Three Faction pardoned her of her past crimes.

Trivia

 * The families of the 72 Pillars are named after the 72 demons from the Ars Goetia, one of the five books of The Lesser Key of Solomon, also known as the Lemegeton.
 * Belphegor and Abaddon are two of the devils associated with the Seven Deadly Sins along with the Lucifer, Satan, Leviathan, Beelzebub, Asmodeus and other devils.
 * It is considered something of a rite of passage for a young up-and-coming Devil to make a pact with a Magician. Young Devils are warned to chose carefully when making such a pact as, if they choose the wrong Magician, it will reflect poorly on the Devil.
 * A Devil can have their Familiars hand out fliers instead of doing it themselves for Devil's jobs.
 * In Volume 8, Rias states that in the past Devils 'did' steal souls, however they no longer do such serious things.
 * In Volume 14, it was revealed that all the current Heads of the Devil households will leave everything to the next heir and enter their retirement after passing the household onto them.