[Theory] Kayle and Morgana are the female Darkin
Before you say "How can they be the darkin if they are 2?" or "Morgana can't be a darkin as she has no weapon!!", hear me out.
We have seen 3 ways for Darkins and Humans to coexist in the same body
Darkin Domination
Human/Darkin Domination
Strife for control
As I see it, there are 2 that still hasn't been shown
- Cooperation between Human and Darkin to become way more powerful
- Separation, what was once fused, manages to become two again
"How can they be the darkin if they are 2?"
I think that both 
and 
used this last method, the Darkin took hold of a fair maiden , yet she was far too powerful, more than Morgana expected, and over time, not only she resisted, but found a way to separate. What was left was a golden woman that hides twisted Darkin flesh under her armor, and a wicked witch whose body is almost fully Human, but needs to rejoin with , as she still holds half her Darkin Power
"Morgana can't be a darkin as she has no weapon!!"
Right, Morgana has no weapon but... Kayle does and as we saw with Dark Assassin , the Darkin powers seems to be kept both inside the weapon and inside the body, so while Morgana has half the power on her body, Kayle holds it inside the Darkin Sword, from where she draws pure Darkin energy that under her control becomes pure light (same as DA 's becoming Shadows)
