The civil war would be a font of dead souls and artifacts she could be tethered to. I am thinking of a native american girl from one of the tribes, who wants to find the answer to the murders any way she can. She would be a 20-25 something at the time of her death and would still have her fears and human flaws,  I want to steer away from the " Omniscient guiding spirit" vibe and flow more towards the girl who desperately wants to be helpful and can be used to pop into places to get information but she is clumsy and sometimes gets stuck in walls.