Herschel Walker has always been something of a puzzle. As difficult as the star running back was to bring down on the field, it was harder, still, to figure out what made him tick. I told somebody once, 'You don't want the Herschel that plays football ... babysitting your child," Walker told ABC News correspondent Bob Woodruff. "When I am competing, I am a totally different person.'" He means it literally. For the first time, the 46-year-old former professional football player reveals in a book ..., Breaking Free: My Life with Dissociative Identity Disorder, that he has a rare and controversial mental illness called dissociative identity disorder - or D.I.D. - formerly known as multiple personality disorder. "I had it the whole time, I just didn't know what it was," Walker said. The athlete who played 15 seasons of professional football in the NFL and USFL and pushed a bobsled for the 1992 U.S. Olympic team in Albertville, France; the family man who married his college sweetheart; the man who once danced with the Fort Worth Ballet; the business man - Walker says none of those guys were him. Not really. Those were his "alters," he says -- alternate personalities. The disorder usually has its roots in childhood trauma. "I was a fat little kid with a speech impediment," Walker told Woodruff. "I used to get beat up, not just picked on." Walker's therapist Jerry Mungadze, said he met Walker's alternate personalities, or alters, in therapy. Walker and Mungadze believe the disorder actually helped Walker - who started for a number of NFL teams, including the Minnesota Vikings and the Dallas Cowboys - succeed on the gridiron.