Human

Humans are not deeply concentrated on in High School DxD, but they do play an important role in the Devil's ranking system.

Summary
After the Great War that 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 consist of humans that are decendants 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, normal humans that have learned magic from other religions/factions. It is said that the current magic system used by the humans was developed by Merlin, the legendary wizard from Arthurian legend. The only named Magicians in the series are Georg and Le Fay Pendragon, both whom are from Khaos Brigade. The Khaos Brigade also have many other 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 have a Magician Faction.

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.

Magicians and Devils also form a pact together, which is used to make a name for both the Devils and Magicians.

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 kills 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 ressurrected by Rias as a Devil, and Dulio who retained his Zenith Tempest Longinus after being angelized by Michael.

Holy Swords and Demon Swords
Humans are also the only species capable of wielding 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 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.