Her markings glow in the dark due to her drinking glow sticks. 

Why does she drink glow sticks? A young child addiction along with that she is obsessed with glowing stuff so when she found that drinking it can make her glow she began to drink it and to numb out the bitter taste she mixes it in with drinks. Also, I've tasted whatever the liquid is in glow sticks and uh... it's pretty bad and it was just like a drop.

 Anyways what demon is she? She's a radio demon mix her voice and most of her powers are THOUGH the other demon part of her is a shape-shifter demon, she can't fully shapeshift only her arms can so she doesn't need to carry around weapons SHE IS THE WEAPON