Clint Eastwood as Frankie Dunn, a gruff but well-meaning elderly boxing trainer. Hilary Swank as Mary Margaret "Maggie" Fitzgerald, a determined, aspiring boxer trained up by Frankie Dunn. Morgan Freeman as Eddie "Scrap-Iron" Dupris, Dunn's gym assistant; an elderly former boxer, he was blinded in one eye in his 109th, and last, fight. Jay Baruchel as Dangerous Dillard Fighting Flippo Bam-Bam Barch or "Danger", a simple-minded would-be boxer. Mike Colter as "Big" Willie Little, a boxer whom Dunn has trained for years. Lucia Rijker as Billie "The Blue Bear" Osterman, a vicious, ex-prostitute boxer. Brian F. O'Byrne as Father Horvak, the priest of the church which Dunn attends, who can not stand Dunn. Anthony Mackie as Shawrelle Berry, an overzealous boxer and frequent tenant of Dunn's gym. Margo Martindale as Earline Fitzgerald, Maggie's selfish mother. Riki Lindhome as Mardell Fitzgerald, Maggie's welfare-cheating sister. Michael Pena as Omar, a boxer and Shawrelle's best friend. Benito Martinez as Billie's manager Grant L. Roberts as Billie's cut man, (trainer) trained Hilary Swank off screen for her Academy Award-winning role Bruce MacVittie as Mickey Mack, a rival of Dunn. David Powledge as Counterman at Diner Joe D'Angerio as Cut Man Aaron Stretch as Himself Don Familton as Ring Announcer

Who played the female boxer in million dollar baby? OK. Let's think. My stream of consciousness:
The relevant information to answer the above question is: Hilary Swank as Mary Margaret "Maggie" Fitzgerald, a determined, aspiring boxer trained up by Frankie Dunn. The answer is Hilary Swank.