Margaret was born a human wizard. She was apprenticed to her father, Ebenezar McCoy.
At some point in her life, Margaret stayed with Lord Raith, and gave birth to his son Thomas. When her son was about five, she escaped Lord Raith, abandoning Thomas to the mercy of his father, who had killed all his other sons. The next year, she met Malcolm Dresden. They married, and had a son, Harry Dresden. She died just after giving birth to her son Harry; originally, Harry believed she died of complications from childbirth, but later learns that she was killed by Lord Raith's entropy curse. In turn, she cursed Lord Raith with her death curse. Margaret was over a hundred years old when she gave birth to Harry and died.
She was renowned for her contacts within the Nevernever among the Fae—becoming known as Margaret LeFay.
Margaret knew more Ways through the Nevernever than perhaps any other mortal magic wielder. She recorded this knowledge in a red stone which was designed to rest inside the pentacle amulet Harry now possesses. The stone would impart the weilder with full knowledge of all the Ways Margaret herself knew, including their counterpoints in the mortal world. Harry acquired this stone in Changes from his godmother Leanansidhe, who was entrusted to give it to him when he was ready.
Margaret also strove for the Council to include more laws that would prevent injustices such and wizards bilking people out of their money, intimidate them, steal property and wealth, or destroy property. According to Luccio, the consequence of changing the Laws of Magic is that the Council would end up intervening in human affairs and ultimately into political human affairs which would cause them in order to take the side of one country over another where there was percieved injustice. Seeing how the White Council had members in all countries, and that the wizards had no better idea of who was right or who was wrong then any other human, this would have been a disaster. And it would force members to turn on their own. The Council would descend into civil war and likely break apart. Then there'd be no protection for humanity against magic or the supernatutal world. The Laws are not about justice, they are about restraining power.
She consistently attempted to prove how limited the Laws of Magic were by skirting them and pointing out the areas of "Grey Magic" as she called them and questioning their legitamacy. The Wardens were tasked with keeping an eye on her. Luccio thinks she was shortsighted about the Laws. 
She was called Margaret LeFay because she was renowned for her contacts among the Fae in the Nevernever. She knew more about the Ways than any mortal Practitioner, She could be in Beijing at breadfast, Rome at lunch, Seattle for supper and stop for coffee in Sidney. Even few of the Fae knew as much.
Anastasia Luccio describes her as a complex woman: brilliant, erratic, passonate, committed, idealistic, talented, charming, insulting, bold, incaustious, arrogant—and shortsighted too—as in her attempts to change the Laws of Magic. She'd never been able to restrain her tongue.
Harry sees his mother's portrait at the Raith Estate. She has dark hair, dark eyes and sharp features, attractive in a striking, intriguing sort of way. Her head had an arrogant tilt and she wore a self-amused smile. 
Harry sees his her again in the Soulgaze with Thomas: she is nearly six feet tall in flat shoes, and ws more beautiful in motion than she could possibly be in a still image. 
Notes In the SeriesEdit
In Blood Rites, right after Thomas Raith shows Harry the portrait of their mother, Margaret LeFay at the Raith Estate and tells him they have the same mother, and they're are brothers. Harry doesn't believe him and thinks it must be a con. Harry and Thomas Soulgaze. During that soulgaze, Margaret talks to her son Harry. First that Thomas will not surrender to him demon, he's of her blood. They hold hands. She gives Harry a gem of knowledge and touches his face, then says she loves him and to tell Thomas she loves him. Harry is overjoyed, he had seen his mother with his Sight, heard her voice, seen her smile and he'll never forget a detail. 
In Turn Coat, Anastasia Luccio discovers by accident that Thomas is Harry's brother. That Margaret had been involved with Lord Raith. Ana tells Harry that she knew his mother because the Wardens were tasked with keeping an eye on her being considred a threat to the Laws of Magic. She tells Harry what she remembers about his mother.
- ↑ 1.0 1.1 Blood Rites, ch. 21
- ↑ Jim Butcher Mysterious Galaxy Q&A @8:15 of youtube video
- ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Turn Coat, ch. 28
- ↑ 4.0 4.1 4.2 Changes, ch. 15
- ↑ Blood Rites, ch. 38
- ↑ Blood Rites, ch. 20
- ↑ 7.0 7.1 Blood Rites, ch. 21
- McCoy-Dresden family
- Margaret Angelica
- House Raith
- Justin DuMorne
- Margaret’s jewel
- Objects of faith
- The Ways
- Laws of Magic
- Entropy Curse
- Bloodline Curse
- Death Curse
- Raith Estate
- Grey Council