What is the Zoroastrian concept of the guardian spirit?

The Zoroastrian concept of the guardian spirit is the Fravashi.

Fravashi is an Avestan term for an individual’s personal spiritual counterpart or protective spirit. Zoroastrian tradition can associate a fravashi with a person who is dead, living, or not yet born. It is not identical to the urvan, often translated as the soul: in one traditional account, the fravashi sends the urvan into the material world to participate in the struggle between good and evil.

Fravashis are portrayed as helpers and protectors of creation and of those who honor them. The Frawardin Yasht, also known as Yasht 13, is the principal Avestan hymn devoted to them. Later tradition connects them with the Faravahar, the famous winged symbol, although identifying that symbol specifically as a fravashi is a later interpretation rather than an explicit physical description in the Avesta. The word should not be confused with Atman in Hindu philosophy or the Egyptian ka and ba, which belong to different religious systems.