Here is a design of Haruka's human form before she turned into a demon.

Her gameplay could be that she starts as a normal human and you help her turn into a demon because she wants to get strong
or She starts with being a demon and you help her get Blue Lily flower in her exploration event to turn her back into a human.
Most probably it'll be the second option.
She was in-charge of the Armory in the battle between Queen and Evil Maiden, when clearing out dead soldiers' armors she gets infected with Evil maidens blood which slowly turns her into a demon but her Elf friend helps her keep sane.

let me know what you guys think about her design.