Not to be a downer, but... do Reds leave ghosts? Susan turned before she died.
If they do leave ghosts... well, Harry's going to have an interesting (and short) life, considering he killed literally all of them, and those ghosts might have a grudge...
I will say that I don't think it was Mab. I don't think she would've done anything so nice as comfort him as he died; I'd imagine she was quite pissed. I'd buy Lea, though; she always seemed to care about him, in her way, and she knew about keeping him alive on Demonreach.
Maybe Bonnie? As an amalgam of Kim Delaney, Murphy, and Maggie, her voice would be familiar, but not, you know, readily identifiable, cuz it's a mix.