Faith is?
Well, it's really just my headcanon but basically:
We know that Murphy was a beat cop in Restoration of Faith, when she first met Harry and fought a Troll.
We know that Murphy was 'promoted' to SI because she wouldn't play politics.
We know that Faith's family are a bunch of Rich Assholes that tried to frame Harry/Ragged Angel Investigations for Kidnapping just to save face/a few bucks.
So theory: When Murph brought Faith in she backed the girl's statement about running away and not being kidnapped, and in retaliation the Astor's got her shunted over to SI because they're rich, petty, assholes.
So yeah, Faith got Karrin sent to SI.
(Although you could also argue this means that Murphy career going sideways was all Harry's fault.)