I think Demon!Rumi HCs in the fandom tend too much towards the bestial and don't take advantage of the fact she's the daughter of a Jeosung Saja, the psychopomps who carry the names of the dead on crimson cloth.
She's always just a little cold. Not corpse cold, but just a little cool. She doesn't overheat easily, and she's fine at temperatures most people would find uncomfortable (Helps with the long sleeves in summer. She's genuinely fine with it) (She has definitely shocked Zoey and Mira awake by surprising them with cold feet)
When she goes "full demon" she doesn't have horns or a tail, but her lips turn the blue of hypothermia, her extremities turn the purple of frostbite, and her gums recede ever so slightly, making her fangs ever more noticeable.
When she wants to be, she can become very, very quiet. Part of this is undoubtedly her hunter training, but sometimes it's as if she simply doesn't create noise.
The teleportation does not help this.
She finds that she always has an inkling of when someone near her is going to die. Sometimes it's a cold sensation in her gut, sometimes it's an unnatural shadow dogging their feet, sometimes it's a series of Final Destination style omens only she can make sense of.
And this isn't even accounting for how her Demon heritage interacts with her Hunter heritage (See how I didn't even mention her patterns)
