Angels are the servants of the Gods Above and the Heavens' equivalent of Devils. They are believed to be of a set, unchangeable number and are divided into Choirs.
Not much has been revealed with regards to their origin or purpose. As an Apprentice, Masego describes them as "[being] of Creation, but not in Creation.", implying that they exist in a separate realm. It's likely that in order to summon an angel a set of specific conditions need to be met, such as was the case with Hashmallim.
They aren't indestructible or invincible, they can be captured and their will can be overcome. Nonetheless, they are very powerful and wield considerable influence. According to Akua, "The last time a Hashmallim touched the world, three hundred thousand people picked up a sword and fought until they reached the capital of the Kingdom of the Dead.”
Heroes, such as the Lone Swordsman, can be aligned with a Choir, though it's not a requirement.
The Choirs are used to categorize angels depending on their methods of influencing Creation. Their number is unspecified and an angel can be associated with more than one. Choirs differ in power and are often associated with a particular number.
So far, six Choirs have been mentioned in the story:
Associated with number seven, the angels belonging to this Choir are known as Hashmallim.
This is the Choir which forces mortals to face their sins and atone, almost always by martial service. It is noted by the Lone Swordsman that this choir is unforgiving - despite providing him power as well as an enchanted weapon, he will still face judgement for his past crimes.
Five out of ten crusades have been caused by Contrition. Out of these, the first is most notable (and the only one that was successful) as it overthrew Dread Empress Triumphant and destroyed the Tower. Crusades five to nine were also influence by Contrition though they were all repelled by the Dead King.
When the Lone Swordsman tried to summon Hashmallim in Liesse, the ritual was going to take "seven times seven hours" and its influence was going to be felt across forty-nine miles. It was going to be the second time Hashmallim would touch creation (the first being in the seventh crusade).
Eleanor Fairfax, the originator of the Fairfax dynasty ruling over Callow was said to be "touched by Contrition".
Ruled by Seraphim, this Choir is closest to number six.
The choir most concerned with ensuring that mortals who go against the will of Heaven are brought to justice.
Humans touched by the choir of Judgement accept that the laws of man are flawed and arbitrary. They accept that to approach justice is to submit to the inscrutable will of Heaven, and to be weighed and found wanting is to be smote. Burdened by an unknowable mandate, they ceaselessly struggle to bring about Fate.
Those who follow this choir are usually called if there is a specific enemy Gods Above believe must be eliminated. The followers believe that only the Heavens can judge whether an individuals deserves to live or die and therefore absolve themselves of making a choice. Heroes in this choir are manipulated more than the average hero associated with a choir. This is most likely due to choir only accepting one hero to do there bidding at a time. They are trained in seclusion and have their aspects adjusted to counter a specific villain.
Currently, Hanno is a hero associated with this Choir.
The choir which values heroes that bear the burden of others.
In embracing the Choir of Endurance a hero will suffer physically, mentally, and will still be asked to serve even more. They have only revealed themselves to the Stalwart Paladin and have been noted as one of the lesser choir's as their warriors are never the leaders of a band but prefer supportive roles.
The Choir demands a great deal from their Heroes thus self-sacrificing individuals, such as Iason, are called upon to serve.
The Choir of Mercy does not concern itself with the sins of man except to comfort struggling souls; those touched by Mercy spend their days alleviating suffering and seeking to lessen the hardships of others.
Filled with empathy for their fellow man, those touched by angels of Compassion are unable to raise a hand against their fellow man. Grey Pilgrim belongs to this choir.