Nah, the Repentia are their own thing, as I keep having to tell people.
No idea who the Penitent should be, or what their crime is, maybe that could be a part of it?
Like, not even the Sororitas know who this pretty, raven-haired chick is (cause it's gotta be a chick), and only have various rumours instead.
Maybe she was a planetary governess, or an Inquisitrix, someone at the very height of the Imperial bureaucracy who fell all the way down to the bottom.
Maybe it was just a street rat who happened, through nothing more than being in the right place at the wrong time, to commit a crime so heinous that even the Emperor won't let her find forgiveness through death.
Stuff like that.
Sadly I can't be much help to you about rules, I could never get into that part of the game, but I guess you'd need to have something to reflect that this thing just won't die, and will throw itself into ever more desperate situations in an attempt to rectify that.