[quote=@TheDarkTemplar]
The Enforcers are the main body of the CIR, they act as a police force while the Sentinels take on the more high risk missions. Since Ace is a mercenary she could be assigned by the CIR to work with Aaron but not as a Sentinel but more as a placeholder until he is reassigned
[/quote]

Hmmmmmmmm.... Could she not simply just apply to be a Sentinel?..... Wait that might not work.... To apply she'd have to go through years of official 
training and be an official agent.... Uuuuuugh... This is so complicated.....

Edit: I edited my first post. Basically, I was just saying how the work Ace did was taking on violent gangs that were proven too difficult for the Enforcers to handle. So in a way she was doing work that was similar to the Sentinel's work.