In Islamic theology, a Shaytan can live among human beings or Jinn, for these two beings alone have been bestowed a "free will" to choose between obedience and disobedience of their lord among all his creations. They incite both with evil suggestions and tempt them into sin. All those among the Jinns and the human beings who follow the mission of disobedience of Iblis, the first Shaytan are all termed Shayateen too.
Shayateen are also "whisperers" who speak into the hearts of men and women alike to push them into doing sinful deeds and straying off the righteous path. This, and the Nafs "self which is inclined to evil" are the sources where the desire to sin comes from, according to Islam. There is a supplication in the Qu'ran which is aimed to fight off these temptations.
Shaytan is a recurring character in the newly introduced Womb of Grief. He observes the progress of both the protagonist and Alex, and slowly falls in love with the latter. If the protagonist completes the Womb of Grief but chooses not to help Alex change the future, Alex empowers herself with the Third Fruit and attacks him alongside Shaytan and Amon.