The idea for this one comes from our mythology about our versions of Angels. Despite working for Zeus, the angels were all created by a universe Goddess called Aurora (although they are unaware of this fact, thanks to Zeus and his need to rule them with absolution).

Once created, the baby angels are placed inside of eggs and dropped from auroras, where The Keeper collects them. The eggs all start off as shining white, but as they develop over the years, they grow trees on the tops, who's knotted roots deliver nutrients and life energy through the shell. When the tree finally blooms and the shell darkens, it means it is time for the Angel to hatch, and the Keeper helps them along. Very, very rarely, two angels hatch from a single egg, and are considered siblings.

The Keeper is genderless, although not technically an Angel like their charges. Good at teleporation and illusion magics, the Keeper is -not- a fighter, and so cannot defend against Zeus' incessant need to take the newly hatched Angels to be brainwashed and trained to serve as his warriors.
Once an Angel is fully grown and the personalty has been evaluated, Zeus assigns them titles. 'Michael' 'Gabriel' 'Metatron', etc... they are kept ignorant of the fact that once Zeus wears out one version of the Angel, he discards them to rot away under Olympus, and replaces them with a newer model taken by force from the Keeper.

Having found out about this unsightly and blasphemous act, Lucifer challenges Zeus, only to be cast out and forced to Fall, where he spends the next several decades trying to kill Zeus and show the other Angels what has happened to their former comrades. Lucifer hatched as one of the most beautiful of angels, but he saw far too much truth behind closed doors, and began to turn bitter. It wasn't until after Michael cut his wings and he retreated below Tartarus that the darkness of his hatred began to corrupt him and make him cold and sly and angry, eventually leading to the creation of his Sins.

The Keeper themself is a peaceful creature, who's only concern is the tending of the angel eggs. In their belly swirls a sphere of water, which can be smooth as glass when they are calm, or turn to a violent and turbulent storm when upset. Either way, their only concern is to tend the eggs and the trees growing from them, as is the duty assigned to them by Aurora, and they have no intention of getting involved with the wars between Heaven and Hell.

Although it breaks their heart to see the angels treated with such cruelty.