This information is mostly derived from Henry Corbin’s Cyclical Time and Ismaili Gnosis. I do not endorse all the below doctrines.
Ahura Mazda dwells in endless light, which is His own Omniscience and Goodness. This Omniscience and Goodness is identical with Dîn, ‘the eternal religion’ or revelation, which is in turn identical with the fravashi. The fravashis are the celestial archetypes aka angels aka Yazatas which watch over everything and constitute the innermost aspect of every human being, acting as our tutelary angels in this world and becoming fully united with us in the hereafter. They are also said to be the innermost essences of everything, including Ahura Mazda, though these non-human fravashis are mentioned less often. The fravashis themselves are said to have fravashis. Some say that there is an indefinite series of fravashis of everything, including Ahura Mazda.
After the fravashis were created, they were given a choice between remaining in Heaven and being incarnated on the Earth to do battle with Angra Mainyu. The word fravashi has the etymological meaning of “those who have chosen,” according to the Iranologists Henry Corbin and Harold Bailey. The fravashis invariably choose to be incarnated. At the time of deliberating about this choice, they are said to partake in Ahura Mazda’s Omniscience, so they know exactly what they are signing up for.
The fravashis of the righteous who have passed away are said to be able to assist those who ask for their help in this world.
Farvarin Yasht 13:70 says (in translation), « They, the Asha-abiding fravashis, come to assist those who are beneficent and not hurtful or offensive. To them, the fravashis will assuredly come flying like birds well-winged. »
For information on Sufi expressions of the idea of the Fravashi, see: http://www.chishti.ru/heavenly_witness.html