Ma'at is the symbol of order and harmony. We don't really see Ma'at personified as a god, but everyone's always talking about "Ma'at this, Ma'at that, blah blah blah" so we figured he/she/it deserved a character section.
One place where Ma'at is personified is in the scales in the Hall of Judgment in the Land of the Dead. The way the scales look gives us a sense of what's wrong with the world: "The center of the room was dominated by a set of scales—a black iron T with ropes linked to two golden dishes, each big enough to hold a person—but the scales were broken. One of the golden dishes was bent into a V, as if something very heavy had jumped up and down on it. The other dish was hanging by a single rope" (28.85).
According to Anubis, the scales are broken because Ma'at is weakening. This is a bad thing for a number of reasons: the souls of the dead can't be properly judged, so they can't proceed to whatever's next in the afterlife, and if Ma'at gets weak enough in general, chaos could triumph and destroy the world.
Another clue to Ma'at's relationship to the rest of the gods comes from Anubis: "[T]he Hall of Judgment is also called the Hall of Ma'at. It is meant to be the center of order, a stable foundation. Without Osiris, it is falling into disrepair, crumbling" (28.170). We're going to go out on a limb here and say that just as Apophis is sometimes embodied in a serpent, Ma'at is embodied in the scales in the Hall of Judgment. If gods can assume any form, from animals and humans to buildings, why not?
Since the scales are repaired in the book's end, after Julius has taken the throne of the Land of the Dead as Osiris, we're thinking that yeah, Ma'at is embodied in the scales, since their repair shows that order has been restored to the world. At least part of the way. Hey, you've got to start somewhere.