The Angel, A, is the most prominent of the angelic beings. They almost exclusively appear in the service of one of the deities, and appear in large numbers on the Astral Plane; you can also obtain a guardian Angel as a minion when you arrive on the Astral Plane if your alignment is sufficiently high and you are not wearing a ring of conflict.
If you are lawful, randomly generated Angels will be generated peaceful. In 3.6.0, one third of randomly appearing Angels will be minions of a random god from your pantheon; usually they will be peaceful to co-aligned players, but some will be made "renegade", peaceful to cross-aligned players instead.
Angels' melee-range magic missile attack ignores both reflection and half spell damage, but deals no damage if you have magic resistance. The missiles they fire at range, however, are ordinary magic missiles. In the message "The Angel zaps you with a magic missile!", it says "zap", but the Angel doesn't use a wand of magic missile.
Angels are generated with an non-cursed erodeproof large shields with a 1/4 chance of the shield becoming a shield of reflection and blessed erodeproof long swords with a 1/20 chance of the sword becoming either Demonbane or Sunsword (If they don't exist yet). The tame guardian angel you get when entering the Astral Plane gets slightly better gear: his weapon is blessed and its enchantment raised by 1-4 points (if it's under +4), and if he didn't get a shield of reflection he will get an amulet of reflection.
He answered and said unto them, he that soweth the good seed
is the Son of man; the field is the world, and the good seed
are the children of the kingdom; but the weeds are the
children of the wicked one; the enemy that sowed them is the
devil; the harvest is the end of the world; and the reapers
are the angels. As therefore the weeds are gathered and
burned in the fire; so shall it be in the end of this world.
[...] So shall it be at the end of the world; the angels
shall come forth, and sever the wicked from among the just,
and shall cast them into the furnace of fire; there shall be
wailing and gnashing of teeth.
[ The Gospel According to Matthew, 13:37-42, 49-50 ]