Marci, also spelled Marcy, is one of the Alphas. She first appears in Fool Moon and is first named (as "Marcy") in the short story Aftermath, with all subsequent works referring to her as "Marci".
Marci is below average height, yet taller than Murphy and very thin, with oversized glasses. Her mouse-brown hair is thin and straight, hanging to her chin cut evenly all around. Marci looks like she can be blown over by a strong wind.
Marci temporarily leaves the Alphas after earning her degree and sets off to make a life in the regular human world.
In the series
In Fool Moon, she's in the back of the van that Susan Rodriguez is driving the Alphas in, described as having mousy-brown hair. William Borden reminds Murphy that she was in the pit with them on John Marcone's estate during the Loup-garou attack. Tera West comforts the mousy-haired girl in wolf form.
In Summer Knight, as she is getting anti-glamour ointment applied, Marci (still unnamed, and described as petite, with mousy-brown hair), holds a dress to cover her naked body and complains that Andi has been impossible all year because of her new slim and beautiful looks.
In Aftermath, Marci and Andi Macklin have been keeping the pregnant Georgia company while Billy is out of town. When Andi and Georgia are kidnapped, she, Karrin Murphy, and Billy free them. It is also mentioned that Andi and Marci "had kind of a thing in school", but it didn't work out (she would later get back together with Andi, along with Waldo Butters).
In Ghost Story, Marci attends a meeting of the Chicago Alliance. Harry Dresden recognized her from the original Alphas. During the meeting, when Felicia Raith threatens to reveal the location of two Swords of the Cross, she positions herself to pounce on her.
In Peace Talks, Harry Dresden is shocked to find that Marci and Andi Macklin are both in Waldo Butters' apartment, with Marci apparently having joined Waldo and Andi's previous relationship to make it a three-way.