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.
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.
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 only named magicians in the series are Georg, Le Fay Pendragon 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.
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.
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 Boosted Gear after being resurrected by Rias as a Devil, and Dulio who retained his Zenith Tempest Longinus 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.
Holy Swords and Demon SwordsEdit
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.