Following ComradeNarf's theory, I would not be surprised at all if Angel and Scratch ran in similar circles and Angel posted bail (most likely for car-jacking or a similar crime). More specifically, Angel's parents posted bail. I know lots of late teens and 20somethings with parents who would (and, in some cases, have) posted bail for their kids to keep them out of jail. I would bet that whatever put Scratch into prison was not directly associated with Angel (or, at least, it wasn't his fault she got put in the slammer) because if you were put into prison you would remember the person that got you in there. You basically have months/years to think about what happened, I would guess you would not forget.

Also, Angel's parents might have even forced him to go to Officer's school (to "repay" them for bailing him out), they might have thought, "Hey! It's just like Military School! It will straighten that kid right out!". And that's why he went from a "life of crime" (albeit petty crime) to a Officer in the Reserve (which also explains why he's in the reserve straight out of Officer's School. Most people wouldn't go to school to be an officer and then say they don't want active duty).

To summarize, Angel and Scratch ran in similar circles, Angel may/may not have been busted and his parents posted bail, Scratch got put in the slammer, Scratch faintly remembers Angel (which leads me to believe it was not Angel's fault she got jailed) and Angel's parents made him go to Officer's School to "straighten" him out. He graduated, and the story picks up from there.