Guys, I think I figured out who Leblanc is

So a while ago Morde new bio was released and it contained this piece of info: >... a coterie of sorcerers slunk forward and sifted through the ashes, gathering up Mordekaiser’s armor and bones. They had been promised great power for their service, but had not foreseen how they were to be rewarded. With newfound mastery over the necromantic arts, Mordekaiser gifted the sorcerers with undeath, trapping them between life and death. They became vile liches, living corpses cursed to serve him until the end of time.

Now, generally, when talking about liches we think of pale people who cannot die and use dark magic. Doesn't that sound familiar? Doesn't it remind you of anyone imortant from the new Noxus lore? Thats why she is called "The Pale Woman". It makes so much sense!

Here's a part of LeBlanc's new bio: > Uniquely skilled among the revenant’s inner circle before she betrayed him, the sorceress pledged to neutralize the source of his power

This just ties everything together. The inner circle is the group of these liches that serve Morde and they are also the only ones who know about his source of power. Mystery solved. TL:DR LeBlanc is a lich