Demons are predominant and intolerable lifeforms that were originally dependent to the Demon World until they found a way to enslave that of the Human Realm. Despite their actions, Sparda made it no longer possible for them to cross into the Human Realm so Demons have released the seven sins to tempt them into choosing the path of the demon which creates the new generation of demonic life forms.
Those now infesting the human realm are those who have forsaken themselves for much greater powers then humans and have used them to take the Human Realm for themselves with the resistance of the Afterlife's Human Organization; The Order.
Demons originally were born of the Demon Realm and were ruled over by their emperor Mundus. Mundus eventually decided to try and conquer the Human Realm with the demons great power but was halted by the infamous; Dark Knight Sparda who stopped them and sealed the gate between both realms. Demons then resorted to tempting humans of their own realm to choosing the path of greater power by forsaking themselves with fully negative Kharma.
Now Demons are bron from Human hosts who have chosen to become more Demonically Powerful. Now Demons are able to take the Human Realm for themselves but are still met by high resistance and are forced to concealing themselves in the forms of government and media to control and enslave the human race and further tempt them into fully becoming demons.
The demonic tendancies control the mind of the human and grant them great powers such as the ability to weild Dark Magic or Devil Arms to combat resistance. Although Demons have all this power they are unable to live forever and once they die they have no hope for the afterlife and therefore are erased from existance. Not all Demons are forsaken by this because the demonic tendancies can be overcome by a strong mind or a hybird born child of both that of Demon and Human.
Most demons are generally very selfish and cruel, and often seek to increase their own power or influence. Very rarely do demons ever show remorse or pity, particularly when regarding humans. On the other hand, some demons are able to love and care, and decide to follow Sparda's ways or live in the human world.
Demons are clearly powerful beings that can easily overcome the majority of humans who have not taken up classes. Demons may be able to take the form of a normal human but they can mutate in many different forms of different shapes and proportions. Their most dynamic ability is their innability to age or die of old age but if they are murdered their abilities can be harnessed into weapons known as Devil Arms. Although a Demon does not actually have to die in order to create one, they can split themselves in order to create one aswell, harnassing their powers within.
Demon abilities are normally superhuman strength and speed, aswell as invulnerability or the ability tofly or teleport. Demons of higher strength rely on Dark Arm's such as Dark Magic to control the elements of the earth such as earth, fire or lightning.
A Demon's most powerful ability is known as the Devil Trigger; it is a rare and powerful ability that unlocks a Demons full potential by drawing power directly from the underworld. A Devil Trigger is actually more easily mastered by Human Hybrids who have a higher understanding and intellegence over normal demons.