It is not Skai.
Like Factor said, Frost appears in the prologue of Emberdark
It is not Rayse, when did his name appear in Elantris?
It is not Adonalsium.
1. This character's name first appears in Elantris
2. This character is first mentioned in The Way of Kings (irl book, I do not think Bajerden knew this character)
3. While this character has been mentioned, they have never actually appeared in a Cosmere book
4. This character has taught a pov character