This hook should be introduced slowly, a glimpse of a street kid here, a fantastic joust there – some of the PCs may even use the “team” as messengers or couriers from time to time, possibly without realizing it.
Sometimes, when an angel finds the rules of heaven too constraining but is not prepared to fall, they find what they find “noble” ways to end themselves.
One such angel has adopted a drew of youth from broken or missing homes. They have no illusions about the world they live in, but are able to find magic within it that others overlook. [Personally, I would pick an elohite outcast, but any choir would do]
As for the angel, zie is dying a slow death; slowly divesting zir forces one at a time to those who make the best leaders and healers in the crew. Eventually the angel will be no more, but zie will have left in zir wake a band of soldiers dedicated to survival and mutual support who owe fealty to neither heaven nor hell.