I came up with a basic backstory to go with her design.
In this world, there are two sentient races: humans and demons. These demons are nothing of the supernatural kind, but instead are a mix of biological parts with technological ones. No one knows if they were a race who experimented on themselves by constantly adding robotic upgrades until their original form was unrecognizable, or if they were created by someone. They are theoretically immortal, being able to replace their biological organs with organs grown in a lab, and naturally producing a liquid which allows their cells to never reject their prosthetics. Most demons choose to not mingle with humans and live in communities on the outskirts of human cities. Cy was abandoned as a baby in such a community. She had severe organ failure, and the demons saved her the only way they knew how, by replacing her failing parts. For her body to accept the replacements, they needed to inject her with the liquid they naturally produced. Most humans died with the liquid's toxicity, but she survived and grew up with the demons, who invented a contraption that allowed her to take the liquid with her instead of needing daily injections.
The liquid is what causes some of her freckles to glow. It's also useful (and dangerous) because it's highly toxic when in contact with surfaces like skin, and corrosive on certain metals, making it useful to fight robots or melt door knobs. Cy only survived this long with that stuff in her body because of her upgrades.
I always liked the idea of a character raised by robots. Well in this case not quite robots, but I hope you like it anyway.
