She was called Margaret LeFay because she was renowned for her contacts among the faerie in the Nevernever. She knew more about Ways than any other mortal practitioner; she could be in Beijing at breakfast, Rome at lunch, Seattle for supper and stop for coffee in Sydney. Even few of the Fae knew as much. For four or five years she lived with Lord Raith, disappearing from all her other contacts.
Anastasia Luccio describes her as a complex woman: brilliant, erratic, passionate, committed, idealistic, talented, charming, insulting, bold, incautious, arrogant—and shortsighted too—as in her attempts to change the Laws of Magic. She'd never been able to restrain her tongue.
Dresden sees his mother's portrait at the Raith estate. She had 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.
At some point in her life, Margaret LeFay 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, Dresden 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 LeFay was over a hundred years old when she gave birth to Dresden and died.
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 Dresden now possesses. The stone would impart the wielder with full knowledge of all the Ways Margaret herself knew, including their counterpoints in the mortal world. Dresden acquired this stone in Changes from his godmother The Leanansidhe, who was entrusted to give it to him when he was ready.
Margaret also strove for the White Council to include more laws that would have not only have wizards governed in all kinds of new ways, but also compel wizards to stop ordinary people from committing injustices. 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 affairs, which would cause them to take the side of one country over another where there was perceived 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 than any other human, this would have been a disaster. It would force members to turn on their own. The Council would descend into civil war and likely break apart, leaving no protection for humanity against magic or the supernatural 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 legitimacy. The Wardens were tasked with keeping an eye on her. Luccio thinks she was shortsighted about the Laws.
In the series
In Blood Rites, right after Thomas Raith shows Harry Dresden the portrait of their mother, Margaret LeFay at the Raith estate and tells him they have the same mother, and they're are brothers. Dresden doesn't believe him and thinks it must be a con. Dresden and Thomas Soulgaze. During that soulgaze, Margaret talks to her son Dresden. First that Thomas will not surrender to his demon, he's of her blood. They hold hands. She gives Dresden a gem of knowledge and touches his face, then says she loves him and to tell Thomas she loves him. Dresden 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 Raith is Dresden's brother and that Margaret had been involved with Lord Raith. Luccio informs Dresden that she knew his mother because the Wardens were tasked with keeping an eye due to her being considered a threat to the Laws of Magic. She tells Dresden what she remembers about his mother.
In Changes, the Leanansidhe gives Dresden Margaret LeFay's jewel that holds all the information she had of Ways. Her knowledge was vast and envied even by the Sidhe. Margaret not only knew where the continually changing Ways were, but where they would be in the future.
Later, Martin states that she is the daughter of Ebenezar McCoy. He was not only her mentor, but also her father. This was the reason that Arianna Ortega targeted Maggie: kill the child, Dresden and McCoy. It's likely that Ebenezar never told the Council that Margaret was his daughter—to protect her from his considerable number of powerful enemies.
In Peace Talks, Lara Raith tells Harry Dresden she knows Ebenezar McCoy is his grandfather, as she used to eavesdrop on Margaret's conversations with Lord Raith. Lara had to "play nanny" for Thomas from time to time, including once when Margaret was visiting her father. After her death, Lara helped her father hang Margaret's portrait in his gallery.