Ma'at - Goddess of Justice
ostrich feather in her hair

Ma'at was the goddess of truth, justice and harmony. She was the wife of Thoth, the god of wisdom, since you need wisdom to find truth and justice. Both Ma'at and Thoth helped at the Weighing of the Heart.

The feather of Ma'at was an ostrich plume. She wore it on her head. The chief judge in charge of the Egyptian law courts was known as the "priest of Ma'at". He began court hearings by wearing the feather of Ma'at. The judge gave the feather to the person who won his case in the law courts.

The pharaoh had to promise that he would follow Ma'at. This means that he would be a just pharaoh, and keep order in Egypt.

http://gwydir.demon.co.uk/jo/egypt/maat.htm