Honestly Either Charlie or Ekko.
We have a cultist who have revealed the failed conversion of Ekko that lends credence to him being scum, and if there's any investigation shennanigans involved with the bravado then this is pretty much as clear as it gets that he is the GF
Charlie, mostly just based off the way he words himself. Sounds like he knows more than he lets on, and well, I just went through that.
Red Night is still on the table, but if he's getting shot, so be it. Not that telegraphing targets ever ends well, but I digress.
I could be convinced on an Orca/ Michelle angle also, but honestly I'm less confident on that one than Ekko/ Charles