REI used to be a completely human, after an accident seriously injured her and had to replace some parts with machines, especially her face was so badly damaged to the point where she had to be implanted with a mechanical face. REI is a special police officer, she is cold and straight to the criminals and sometimes a bit rude to her co-workers and maintainers. She often ignores maintenance, she is frequently damaged and rusted, it can be said that Rei is a bit Tsundere.