Devil

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

Appearance
Devils look exactly like humans, albeit with black bat-like wings that can be hidden at will. It is still unknown if Devils can have multiple wings like the Angels and Fallen Angels. Reincarnated Devils can come in different appearances, such as the case with Tannin, a former Dragon King, and Enku, a Qilin.

Abilities
All Devils have increased physical abilities, such as superhuman strength and endurance, and enhanced senses such as increased hearing range and eyesight. In addition, as they are creatures of the night, Devils have the special ability to see clearly at night. Devils also have the ability of flight through their bat-like wings.

High-Class Devils that have a "peerage" rule over other demons depending on their family's rank and powers, and 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 in higher authority. High-Class Devils also have the ability to turn a human into a Devil when they die through the Evil Pieces system, as seen with what Rias did with Issei.

A common weakness for all Devils is that they cannot go near/get involved with anything that has to do with God, as they can 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 also susceptible 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 the True Longinus (Asia's Twilight Healing, however, is a notable exception).

Ranking
The Devils use a hierarchal system that is split into four different ranks. They are: There is three different ways to raise one's ranking.
 * Ultimate-Class Devils (最上級悪魔 Saijōkyō Akuma): Includes Satans, Great Kings, Kings, Archdukes, Dukes, and Devil clan heirs/heiresses.
 * High-Class Devils (上級悪魔 Jōkyō Akuma): Includes Marquis/Marchioness, Margrave/Margravine, Count/Countess, Viscount/Viscountess, and Baron/Baroness.
 * Middle-Class Devils (中級悪魔 Chūkyō Akuma): Includes Baronet/Baroness and Knight/Dame
 * Low-Class Devils (下級悪魔 Kakyō Akuma): Includes reincarnated Devils. Low-Class Devils can promote themselves to higher rankings depending on their performance.
 * 1) Doing Devil Jobs by granting the clients wish. This is the slowest method.
 * 2) Obtaining great acheivements in times of crisis or situations. This is the fastest but does not happen often in time of peace.
 * 3) The last method is by doing well in Rating Games. This is by far the best method to raise one's own ranking.

Four Satans
The Four 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 are chosen to replace the four original Satans who died in the Great War, and each are named after four original Satans: Lucifer, Beelzebub, Leviathan, and Asmodeus. Therefore the names "Lucifer" and "Beelzebub" are not personal names, but are more like managerial positions.

The current Four Satans are Sirzechs Lucifer (Gremory), Serafall Leviathan (Sitri), Ajuka Beelzebub (Astaroth), and Falbium Asmodeus (Glasya-Labolas). The descendants of the previous Satans – Cattleya Leviathan, Shalba Beelzebub, and Cluzerei Asmodeus – have allied with the Chaos Brigade's Old Satan Faction.

72 Pillars
The 72 Pillars (72柱 Nanajūni Hashira) are a list of the 72 families of pure-blooded Devils (known as Pure Devils). The families representing the 72 Pillars are: Bael, Agares, Vassago, Samigina, Barbas, Valefor, Amon, Barbatos, Paimon, Buer, Gusion, Sitri, Beleth, Leraje, Eligos, Zepar, Botis, Bathin, Sallos, Purson, Marax, Ipos, Aim, Naberius, Glasya-Labolas, Bune, Ronové, Berith, Astaroth, Forneus, Foras, Asmoday, Gäap, Furfur, Marchosias, Stolas, Phenex, Halphas, Malphas, Raum, Focalor, Wepal, Sabnock, Shax, Viné, Bifrons, Vual, Häagenti, Crocell, Furcas, Balam, Alloces, Caim, Murmur, Orobas, Gremory, Ose, Amy, Orias, Vapula, Zagan, Valac, Andras, Haures, Andrealphus, Cimeies, Amdusias, Belia, Decarabia, Seere, Dantalion and Andromalius. Out of the 72, only 34 are still active after the Great War (in reality, there are more than 34 houses that are still active but lost their status in the 72 Pillars due to having human heritage).

Extra Demons
Extra Demons (番外の悪魔（エキストラ・デーモン） Ekisutora Dēmon) are houses of Top-Class Devils who are not part of the 72 Pillars. 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. The 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 and Kuroka, the latter coming from the Chaos Brigade's Vali Team.

Super Devils
Super Devils are Devils with abnormal demonic powers that surpass even the original Satans. The only known Super Devils so far in the series are Sirzechs Lucifer and Ajuka Beelzebub. According to Azazel, there is another Super Devil but has hidden himself.

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.
 * Many of the Pure-blooded Devil clans appear to have unique powers, such as the Baels' Power of Destruction, the Abaddons' power of Hole, and the Phenex's power of Immortality.
 * It is considered something of a right 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.