Nephilim are the other side of the coin to Mages. Mages are humans with a piece of the Divine Awakened inside them and Nephilim are born a Divine Being in a mortal shell. They constantly struggle between their Purpose and their own Free Will.
Nephilim come into the world when an Angel of either generation procreates with a human being. This can be done in a number of ways, not all involving sex (especially for first generation angels). But it always ends the same way: this half-breed is born and will struggle until it is either destroyed or it reaches Ascension and sheds one half of itself to become either fully mortal, or fully angel.
The parents’ choir does not determine what the child will be. This is not discovered until the child’s first Halo, which often occurs in the mid-teens. Purpose seems to play the largest role in determining what type of Angel the child becomes and whether they embrace Grace, Balance, or Malfeasance.