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, their will can be overcome and while they cannot truly die, they can be 'killed', though the killing of an angel does not reduce the strength of the Choir in any way. A dead angel 'corpse' is known as an ealamal in Levant. The only two Villains known to have slain angels are Dread Empress Triumphant, who 'killed' a Seraphim when Proceran Hallowed summoned one to judge her and Dread Emperor Traitorous, who tricked a Hashmallim into perdition at Liesse. Nonetheless, they are very powerful and wield considerable influence.
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, may she never return, and destroyed the Tower. Crusades five to nine were also influence by Contrition though they were all repelled by the Dead King.
Before the Seventh Crusade, the White Knight, disgusted that the First Prince would even consider agreeing to the Dead King's deal of getting 500 children every year for peace on the borders, called on Contrition in Salia. Every person in Salia, around 300,000 people, took arms against the Dead King, launching the Seventh Crusade.
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".
Catherine considers Contrition to be one of the weaker Choirs.
It is mentioned that a Hashmallim were deceived and killed by Dread Emperor Traitorous, with the corpse left behind in Lake Hengest beside the city of Liesse.
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.
The Saint of Swords mentions that Judgement can directly intervene in a fight by smiting Villains, which they demonstrated against the Black Knight in his fight against the White Knight. An act that brought Black to near death and rendered him too weak to finish off Hanno. In Catherine's opinion, Judgement and Mercy are the two strongest Choirs, surpassing even Contrition.
Previously, Hanno was a hero associated with this Choir.
The Seraphim are currently mostly unable to act due to the Hierarch forcing himself into the Choir, which also led to them losing their connection with Hanno, though they still act on citizens of Bellerophon when both the Seraphim and Hierearch are in agreement.
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.
Angels belonging to this Choir are called the Ophanim.
They do not intervene directly in fights, unlike Judgement, but provide various valuable supportive powers instead. Such as intel on the nature of people, their choices and the endings that they will take them down as well as their powers. For example, they observed Catherine in order to gain an understanding of her powers of Winter and Night, informing the Grey Pilgrim of her state. It is mentioned that they whisper to the Grey Pilgrim where he should go in order to alleviate suffering. On other occasions, they have been seen empowering the Grey Pilgrim, allowing him to stay awake even after using massively exhausting miracles and aspects. They also provide some analysis on the nature of power, such as informing the Grey Pilgrim that being caught in the torrent souls massed by Masego against the Dead King would result in death. They also prevented the aforementioned torrent of should from coming near the Grey Pilgrim.
They are mentioned to rival Sve Noc in power. Although they are unwilling to get into a direct confrontation with the sisters, Catherine mentions that they were both older and could be more ruthless than the sisters.
Catherine describes their appearance as, "Under the stars a multitude of tall and thin silhouettes stood, the only marks of their presence silhouettes like a heat shimmer and ever-spinning eyes like wheels of flame. There were dozens and dozens of them, all bent as if in grief."
The Grey Pilgrim belongs to this choir.
Filled with empathy for their fellow man, those touched by angels of Compassion are unable to raise a hand against their fellow man. It is mentioned that healer Named have been sworn to this Choir in the past.
They seem to be based on the angels in Judaism, with Hashmallim, Ophanim, Seraphim and Cherubim all being ranks in the angelic hierarchy of Judaism.