Zelchar, thanks for opening this topic.
My WAG but a considered one - the Leanansidhe is a mantle, just like Mab, Titania, the Winter and Summer Ladies, Kringle, etc. Otherwise, that would make Lea – the most powerful in the Winter Court after Mab - the only high level Sidhe that isn’t a mantle.
And just like those with mantles, the Leananside was mortal once.
And the mortal who has the Leanansidhe mantle is an ancestor of Harry. I think it likely Harry’s grandmother or maybe his great grandmother. (I thought maybe Margaret at one point, and while I don’t rule her completely out, I don’t think the math works even though at times Lea calls Harry “Son.”) This would solve a number of unknowns and curiosities surrounding Lea but it was Lea’s participation in Mexico against the Red Court that pretty much seals it for me. Mab allowed or directed Lea to go - and to indulge herself - even though Mab had earlier openly questioned why she would risk losing her Knight in a matter in which Mab had no interest and, also, would have resulted in hostilities between the Winter Court and the Red Court unless Harry wholly eliminated the Red Court. Why? Because the Red Court’s planned curse to kill Harry and Eb would have killed the mortal wearing the Lea mask. Also, note that in the aftermath of Red Court’s elimination when Eb asks to speak to Harry alone, Eb says “Family business” and Lea smirked at Eb. Maybe that was acknowledgement that Lea knew of the family relationship between Eb and Harry, but I think that’s what Butcher wants us to think. I think it has deeper meaning than that (and Eb doesn’t know a relative is wearing the Lea mantle).
To those who want to cite WOJ to refute this, I consider Jim to be an unreliable narrator when it comes to things that could reveal major plot points in his planned, yet-to-be-published books. He can’t give away critical pieces of the game with so much left so he understandably obfuscates. And lest anyone forget, Butcher is a huge Star Wars nerd and by extension, so is Harry. When Butcher says Harry’s grandmother is dead, I just think of Obi-Wan telling Luke that Vader killed Luke’s father. From a 'certain point of view,' Harry’s grandmother (or maybe great grandmother, even Margaret herself) wearing the Lea mantle is just as dead as Anakin was wearing the Vader mask.