Allies are NPC Sorcerers that you can take with you on Avalon Pacts. You can take no more than two allies with you at once.
There are several ways that you can gain Allies. The most common method is by saving Archfiends, in which case the human that was once the Archfiend will join you. You can also gain Allies automatically in story mode, as well as by completing a specific quest(usually the first quest) in a Fellow Sorcerers Pact. Occasionally, you will come across a dying Sorcerer, and either saving or sacrificing them will unlock them as an Ally. (though in the latter case, you'll first have to pay Lacrima to revive them)
If at any point an Ally is slain or sacrificed, you will be able to revive them so long as you have enough Lacrima.
Affinity and SeparationEdit
Each Ally starts out with three offerings. All allies that come from original Archfiends have the Archfiend's special magic in its S version as one of their starting offerings (the only exceptions are Endeir, Bren and Jessom). As you complete quests with Allies and perform actions they approve of, they will gain Affinity, which is measured by a bar on the Ally's page in the Portraiture. There are four levels of affinity : C, B, A and S. Once you raise an Ally's Affinity enough, that Ally will gain a new offering. When an ally's affinity reaches S level they will gain access to all (6 in total) of their offerings.
However, Affinity can also be lost. Like you, each Ally has either a Divine, Neutral or Dark arm, which determines how they react to your decisions. Divine Allies approve of saving Archfiends and disapprove of sacrificing them. Conversely, Dark Allies approve of sacrificing Archfiends, but disapprove of saving them. If you perform too many saves or sacrifices with a Dark or Divine Ally with you, that Ally may become "Separated" from you, preventing you from using them in quests. Separation can be undone by using Lacrima. Neutral Allies, however, have no preferences between saving or sacrificing, and will gain Affinity regardless.
Notes on affinity:
Dark affinity allies will always sacrifice you upon your death (very inconvenient for single player)
Divine affinity allies will always save you
Neutral allies will listen to your request
Saving your Divine Arm allies, in case if they ever fall in a pact, increases their affinity by a lot more, if not faster, compared to normally completing it. However, it seems that sacrificing Dark Arm allies does not give you the same effect.
Sacrificing allies does not stop them from gaining affinity at the end of the pact.