Humans are mortal creatures of the Corporeal world.
Humans have come up with quite a few interesting ideas about religion and metaphysics. In human depictions of angels and demons, one typical theme is that humans are, for all their humble appearances, capable of greater evil and nobler good than either type of supernatural being. In reality, this is arguably rather inaccurate, or at least misleading. Technically, yes, certain humans can show more good or more evil than the average angel or demon. However, you don't have to walk far in Hell to bump into a demon more intensely and willfully evil than the spookiest human alive. Human arrogance is nothing compared with a Balseraph, and human devotion pales next to an attuned Cherub.
For the most part, the defining (and sometimes saving!) trait of humans is actually their mediocrity. They are part of the Symphony, and sometimes tune in to Good, but this comes and goes. An angel is powerfully bound to the Symphony, just as a demon is powerfully bound to their personal symphony, and escaping that bond even for a moment is rare. Humans, lacking such ties, are very different creatures.
The average human is composed of 5 Forces. They regenerate Essence at noon, and (since they are made up of 5 Forces) can hold up to 5 Essence total. The average human cannot use Essence consciously; instead, it gushes out when they really try to succeed at something.
Some of them, however, have the potential to become Symphonically aware. This allows them to incorporate additional Forces, to use Essence consciously and discriminately, and to use Corporeal Songs. Many of these aware mortals become Soldiers for either side in the War or for a power in the Marches; a smaller minority become Sorcerers, seeking a way to take hold of reality themselves. Both Soldiers and Sorcerers typically have 6 Forces.
Most mortals with Symphonic potential must be awakened; a few rare souls, however, seem to "open their ears" naturally, sometimes believing their gift to be psi or magic. This latter group often includes descendents of the long-Outcast Grigori, the only Choir able to interbreed naturally with humanity.
Humans who die and go to Heaven sometimes want to come back to Earth and keep fighting the good fight. These are Saints. Others try to cut a deal with Hell to avoid death; these become the undead, living well past their normal span, but with a soul that unravels if they're killed. An undead that is finally put down doesn't go to Heaven or Hell -- its Forces simply return to the universe.