Milkette's a Lich.
No more beating around the bush, this is what she is, no confusion. However she's particularly good at hiding it. While most Liches are cold, dead husks of vastly unpleasant demeanour and overly powerful stat buffs and resistances, Milkette's from a different sort of cloth.
Any time you're gonna see her, she's gonna be the same ol' warm little mousegirl you guys love to bother. But if you ever see her like this - an eight-foot-tall monstrosity wreathed in soul-frosting fury and throbbing with unearthly power, then you've either made a very big mistake or she feels like scaring the bejeebus out of you.
Souls are energy, and she puts them into her phylactery like a piggy-bank. Fortunately for (most) you sorry mortal plebs, she's not after your souls. You can keep them for a while. Instead, there's a whole demon hierarchy with new upstarts cropping up every day in the underworld, and a lot of them are way too big for their britches. Better for her to fill said britches with her jiggly undead mousebutt, don't you think? So, she takes that soul power from evil, cruel creatures who probably would flay you no sooner than look at you.
She accepts her thanks for stemming the tide of evil into your sorry little lives in the form of milkshakes and large pepperoni pizzas. Get crackin'.
Art by Norithics