You may assume that Skarlet is 100% human, but the actual amount is much, much less than that. her actual origin began in hell, more specifically as the highest ranking general in the infernal lord's military.

She was feared all across the underworld, known for her immense brutality and the sadistic pleasure she took in cutting down all that opposed her and her king.

What most denizens of the abyss didn't know, was that their ruler was not, in fact, a horrible bloodthirsty demon all the time. Lucifer was a misunderstood being that only wanted what was best for the lands that he controlled.

Skarlet is the result of Lucifer taking all of his violence and wrath and pushing it into a vessel able to handle it without backlash.