You must beat Norma in every chapter to unlock her. In Chapter 3, she is located outside the Hole. In Chapter 4, you must to enter the Station Zone from the South, without a vehicle. She can be found in the Station Zone again in Chapter 5. In Chapter 6, you must exit Industrial Area South from the East. She will approach you in the Town Square area. Beat her there. Finally, go to the Hotel Zone. She will call you and tell you to meet her at the basketball court. Go and beat her there, and recruit her. If you miss any encounters, you'll have to start a new game.