Humans

Humans (人間, Ningen) are not deeply concentrated on in High School DxD, but they do play an important role in the Devils' ranking system.

Summary
Largely residing in the human world, humans are the most common species with the largest population in the series. After the Great War had exhausted all Three Factions, the Angels, Fallen Angels, and Devils all had to rely on humans to continue their generation.

The Khaos Brigade's Hero Faction consists of humans that are descendants of heroes, and each has either a Sacred Gear/Longinus, Holy Sword, Demon Sword, or any combination of the three.

There are also the magicians, humans who have learned magic from other religions or factions. It is said that the current magic system used by the humans was developed by Merlin Ambrosius, the legendary wizard from Arthurian legend. The some magicians are Georg, Le Fay Pendragon, Lavinia Reni and Walburga. The Khaos Brigade have many magicians serving under them, as shown in Volume 4 when they attempted to murder the top brass of the Three Factions. It is revealed in Volume 12 that the Khaos Brigade has a magician faction named Nilrem.

There are also humans who have been artificially created for certain purposes. The "Sigurd Institution" were a group who were trying to create the "true descendant of Sigurd" and succeeded with the creation of Siegfried.

Contracts
Devils need humans to sign contracts with them, which increases their power and standing in the Underworld. However, in order to get the human to sign the contract, the Devil must first fulfill their end of the bargain. These tasks can border on the insane, as Koneko is shown to have cosplayed for some of her contractors, and Issei has been asked to bring a doll to life and make it real by a cross-dressing fan of a series.

A Magician and Devil can also form a pact together, which can be used to make a name for both the Devil and Magician.

Sacred Gears
Humans are the only species that can naturally be bestowed with Sacred Gears. However, the other races, (Fallen Angels, Angels, and Devils) can obtain Sacred Gears by taking them from their human hosts through a ceremony, though removal of the Sacred Gear would kill the host.

Humans can somehow retain their Sacred Gear even after being reincarnated into Angels or Devils, as can be seen by Issei keeping his Longinus; the Boosted Gear, after being resurrected by Rias as a Devil, and Dulio who retained his Longinus; Zenith Tempest, after being angelized by Michael.

Humans have shown that with constant training and fully mastered using Sacred Gear, they can be capable of defeating High Class Devils, Monsters and Demonic Creatures. Wielders of the Longinus have been shown to fight God-class beings when they mastered the powers of their Longinus.

Holy Swords and Demon Swords
Humans are also the only species capable of wielding both Holy Swords and Demon Swords. Like the Sacred Gears, humans can retain the ability to use Holy Swords and Demon Swords even after being reincarnated as Angels, Fallen Angels, and Devils, as seen by Xenovia Quarta retaining her ability to use the Durandal after being reincarnated as a devil and Yuuto Kiba being able to use the Demon Swords that he took from Siegfried.

Magic
Humans have shown the ability to use magic by studying the Demonic Power of Devils, as they are capable of replicating the effects of Demonic powers by using equations instead of imagination like Demonic power does. Humans who learn to use magic are called Magicians. Magicians form their own organizations to further study magic and increase their magical knowledge. Magicians also make pacts special Devil-Magician pacts with Devils, with Devils offering their protection and knowledge in exchange for the unique magical technique that Magicians produce with their research. Making a pact with a famous and powerful Devil is considered a mark of status among magicians as it brings great fortune and success to them.

Trivia

 * Although humans are normally seen as being the weakest among all the species in High School DxD used as an important part in the Devils' Evil Piece Ranking System, there are a handful of humans able to defeat powerful monsters and beings. Among them, there is a small number of humans who're known as being the Strongest Humans (, Saikyō no Ningen ):
 * Cao Cao, the wielder of the Strongest Longinus, True Longinus known the "The Strongest Human" by Vali.
 * Tobio Ikuse, the wielder of the Independent Avatar-type Longinus, Canis Lykaon known as the "Slash Dog".
 * Mitsuya Kanzaki, the holder of the two Longinus, Innovate Clear and Telos Karma known as the "First Dark" and "The Man Who Impersonates God".
 * Arthur Pendragon, the wielder of the Strongest Holy Sword, Caliburn known as the "Strongest Holy Sword User" and the "Holy Royal Sword".
 * Vasco Strada, the previous wielder of Durandal and Veteran Exorcist, known as "The Church's Device of Violence", "The Violence of Heaven", "The Vatican's Evil Killer", "Mr. Durandal", and "True Devil", a human who achieved strength dubbed "Limits of Humanity".