I zeroed in on Faith Astor because of a few WOJ's, some of which have been floating around longer than I have been reading the series. One of them is the Faith Astor will return near the end of the series and bring Harry a case that helps kick off the BAT; a couple others I kinda-sorta remember but could have mixed the details concern Kumori. I think one of them is that we have met Kumori elsewhere in the series under another name, and that 'her identity will hurt Harry'. (if someone has those WOJ's feel feel to chime in and correct me, since my computer ate those bookmarks).

Working from that, I think there is a fair amount of circumstantial evidence: Kumori would have been approximately the right age to attend the soiree in GP alongside her master as an apprentice, and another 3-4 years on in Dead Beat she would be the right age to be a newly minted spellslinger in her own right. When Harry meets Faith in the chronologically first story, she is young enough that she has not hit puberty and her growth spurt; that means if she developed magical talent it would likely have been after, and it is difficult to get a fix on someone's adult height at that age; let alone what their voice will be. That would explain Harry not recognizing Kumori at all, yet Kumori having a genuine care about what happened to Harry's hand and (apparently) warning him off without the say so of her superior.

As for hurting Harry: finding out one of the first people he saved in Chicago developed into a Wizard level talent under his nose, who was exposed to the supernatural and likely at least partly inspired to develop those abilities on account of her interaction with Harry...then getting scooped up and corrupted by some of the worst bad guys around? That might hurt Harry more than anything that does not involve his children, his brother, Murphy, or Michael's family. Having Molly version 0.5 with a terrible family now working for the baddest of the bad guys, except he never saved her, is going to hit Harry right in his little orphan heart.

The last piece of circumstantial stuff is that whatever Faith does will at least partially light the final fuse of the BAT; in keeping with the original noir-ish trappings of the series, Faith showing up at whatever serves as Harry's office at the point as the archetypal femme fatale would be very appropriate. And of all the things that could touch off a final scramble before the BAT, a high ranking member of the Circle offering to turn the equivalent of state's evidence on the bad guys in return for protection and the chance to walk away might be near the top of the list. If Kumori approaches Harry Dresden and offers to tell him everything she knows about the bad guys, that is the 'break glass in case of emergency' moment for the bad guys, exactly the sort of thing that would incite them to jump the gun and touch off something apocalyptic before all the pieces are in place; and give the good guys a chance to steal a win they would have little hope of achieving on their own.

 Faith being Kumori provides a good explanation linking all the pieces together without creating any obvious contradictions I can see; and it does not rely on alternate universes, time travel, or Elaine being evil (I think she will replace Fix as Summer Knight within the next 3 books).

Based on the timeline, Faith Astor would have been around 19 during Grave Peril, which is plenty of time to complete an apprenticeship or come close enough to accompany a master to a formal event. And yes it would hurt Harry, thanks to his overdeveloped sense of responsibility. His saving Faith also exposed her to the world of magic, and if she is Kumori that means that a Wizard level talent, one he had encountered personally, developed under his nose, in his town, then either followed a similar path to Molly-exploring her powers on her own instead of approaching him, then falling in with the wrong crowd-or was discovered and cultivated by dark powers and he never tripped to it. Either way, Harry would view it as a personal failure.

Luccio does not know who she has locked up until she hears Page call a name: "Grevane". At that point Luccio realizes who she has in custody, and Kemmler escapes before she gets back. She also specifically mentions Kemmler was already on the most wanted list; so if she had recognized him right away, I think it is likely she would have explained to Earp before executing Kemmler in the cell, and led the Warlocks on during the standoff in an attempt to survive/kill them as well.