Beings of white wings and any color of skin, eyes, or hair. They come in several ranks, only differing in the power they possess. From low to high, these are: Angel, Archangel, Principality, Power, Virtue, and Dominion. There are three ranks above these six, being Throne, Cherubim, and Seraphim, but they do not manifest on the Material Plane as they can't stand the impurity prevalent there.
One could be a 'born' or 'created' angel. Which happens when a mortal child, or pure hearted mortal, dies. Their soul comes from the maelstrom to a secret sanctum, in which a genderless body grows. It is the last, and most powerful of Celestial relics. One so old that none can remember how it works. Or how to maintain it.
Lastly, the most recent development that came with the Alliance of the Renounced, angels have the option of possessing whatever gender they desire. This includes their choice of being fertile or sterile. As such, there have been a few angels born by natural causes. This, however, began no more than a decade ago.
Another way to become an angel is to undergo one of two redemption ceremonies. One spans the period of a year, in which the pilgrim takes a vow of silence. Then, at the six-month mark, they take on a vow of celibacy and abstinence. All through the year long process, there are fasting periods in which the participant dwells on their sins to have it purified by their guardian angel. Each session is a week longer, until the final month-long vigil. At which point, the ascendee possesses enough angelic grace to survive.
An alternative is to cram everything into a single month. Taking on the month-long fast, as well as the vows of silence, celibacy, and abstinence all at once. While a guardian angel helps the meditating ascendee shed his, or her, sins. Not to mention, teach them how to collect grace. It's an arduous process meant only for the most pious and healthiest of individuals.
While any creature can ascend to angelhood, the opposite is true as well. Given enough corruption an angel can corrupt fully, becoming a demon. Angels and demons are, after all, nearly the same race.
Angels, like demons, age differently than mortals. They become fully adult at ten years of age; seeing as they develop at twice the speed of humans before plateauing. To the point that their 'mid-twenties' seems to last hundreds of years.
Lost to the Upper Planes
Due to their stay in the material plane, and its residual impurity (or Corruption), angels in Shades of Immortality have lost the ability to return to their homes in the Upper Planes. For some angels, returning there is their primary goal in life, of course this is a problem because...
Weakness to Corruption
Where demons cannot handle being in the presence of purity for long, the opposite is also true for angels. Being near someone, or something, truly corrupted will begin to itch. And coming into contact with corrupt things, like hellfire, will burn them. Far worse than it ever could burn a mere mortal.
A 'disease' called Holy Bloodlust has recently gotten out of hand. Enough that it's now public knowledge, though the Dominion Council had kept it a secret even from their own kind until a decade ago.
It's a condition where the degree of a person's purity determines how tolerant they are of 'ambient' corruption. That is, when confronted by a demon, they go 'insane' and won't stop until the demon is dead.
The same could be said for non-demonic individuals. If someone or something possesses enough corruption to trigger someone with HB, then the afflicted angel will stop at nothing until the irritant is eliminated.
Of course, as HB progresses with increasing purity of said angel, the lower their tolerance for corruption becomes; and the less free will they seem to possess. Until they become a mindless creature of purity on a bloody rampage.