Phobos and Deimos were Rei's guardian crows, owned by Hikawa Shrine, where she works as a priestess and were her guardians.
When they were in human forms, they wore leotards, black-winged front bows, and lilac, enlarged, extended back bows. Phobos' color was purple, and Deimos' color was red, both of which happen to be Rei's favorite colors, and Sailor Mars's uniform and bow colors.
In the Stars arc of the manga, it is revealed that they are from the planet Coronis, and that they were acquaintances of Sailor Lead Crow. In the anime, they only appeared as crows, never in human forms.
Phobos and Deimos are mentioned by Sailor Mars in the game as a stunt attack.