Margaret Gwendolyn LeFay Dresden is Harry Dresden's mother, who died in childbirth. Little more can be said about her without spoiling the events of Blood Rites. If you do not wish to be spoiled for that book, read no further.
NOTE: Fans haven't quite pinned down specifically what years these events take place, a system of BSF (Before Storm Front) or ASF (After Storm Front) has been established.
When she begins to come in to her power, barely more than a child, she is apprenticed to Ebenezar McCoy. However, she balks under his overly intense regimen. She runs away as soon as she can get away with it and takes up with bad sorts out of sheer rebellion. "She made a couple of bad decisions, and … and then it was too late for her to go back." One of her associates includes Justin DuMorne.
She takes up with Lord Raith, who tries to snare her permanently. However, she proves to be too strong for him to completely enthrall.
34-35 BSF: Thomas Raith is born.
~29-30 BSF: Maggie escapes Lord Raith. Thomas is five years old.
~28-29 BSF: Maggie meets Malcolm Dresden.
27-28 BSF, October 31: She is killed during childbirth by Lord Raith's Ritual Entropy Curse, but with her death curse, she makes him unable to feed.
It is unclear when she sets up the special soul gaze message for Harry and Thomas.