Harry, Justin and Elaine.

My theory is this.

Justin DuMorne was body swapped by Kemmler when the Wardens fought him. He was killed before he met Dresden and Elaine.

Kemmler adopted Harry and Elaine, knowing the next cycle would pop off, to get himself s Starborn vessel. He intended for it to be Harry, but one must always have an heir and a spare.

When Harry "killed" Justin before the start of the books, Kemler jumped bodies again. This time to Elaine Mallory. 

Harry realizing that the woman he once loved, the mentor he thought he killed, and the monster who shaped his entire life were all the same goddamn person is going be the catalyst to the BAT, because he'll have to kill Elaine to stop a bad guy, much like he killed Susan in Changes.