So what's the story behind her able to become a spirit at will?

She's not able to become a spirit at will, she is one.
I have this whole little.. world planned out in which I have all the different mythologies mixed and added some stuff of my own. Like, there are different kinds of spirits and this kind in particular is closer connected to humans, which gives them the ability to turn into a physical form. They aren't made of flesh and bones, though, but more like something... jelly like that isn't transparent. Which is why she has as much shine on her as he does.

Hmm, last piece made me think that she could based on her appearance, or at least switch between the two forms.

Still the idea is rather intriguing, I'd like to know more about it sometime.

She can switch between them, but they're both spirit-forms, as she doesn't have organs and stuff like that. It's kinda like water turning to ice

your painting skills are so sublime!

Hey thanks, Owly. Maybe you'd like to do a collab sometime?

that'd be really awesome!

I wouldn't mind her to scaring me late night.