When Tracey Miller was 15, she was a leader of the Brixton gang, the Younger 28s.
She would sleep with a knife under her pillow and a gun by her bed. Now, 20 years on, she works with kids, trying to stop them falling into her former life.
She has also published her memoirs, called Sour.
The ex-gang girl says she can’t remember the number of people she has stabbed.
Newsbeat spent a day with Tracey, who says she was once one of the “baddest” girls in Brixton, to reflect on her former life.