Human

Humans
Humans aren't concentrated on in Highschool DxD, but they do play an important role in the Demon Ranking system.

Contracts
Demons need Humans to sign contracts with them, which increases their power and standing in Hell. However, in order to get the Human to sign, the Demon must first fulfill their end of the bargain. These taks can border on the insane, as Koneko is shown to have to cosplay 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.

Sacred Gear
Humans are also the only species than can naturally be bestowed with Sacred Gear. However, the other races, (Fallen Angels, Angels and Demons) can obtain Sacred Gear by taking them from their Human hosts through a ceremony, though removal of the Sacred Gear kills the host.

Humans also retain their Sacred Gear when transformed into Angels, Fallen Angels or Demons, as can be seen by Issei keeping his Boosted Gear / Gauntlet of the Inferno Dragon / Crimson Dragon Emperor Gauntlet after being ressurrected by Rias as a demon.