Mariya Lumen, or Eidolon, is a young human who first appeared in a mobile chat client while Aria and Reed were in the monster world. She is Reed's younger sister, and commonly assists in the joint search for their father. When talking to others about Mariya, Reed hesitantly refers to her as "Headquarters".
She is the person behind the gadgets included in Reed's mechanical arms, and appears to be a young prodigy in middle school pursuing engineering. She shares a residence with Reed in the human capital of Rosebud, and largely depends on him for cooking food and help with homework.
Notably, Eidolon is also a huge fan of the in-universe franchise Galactic Princess Selene.
- The term Eidolon can mean a spectre or a phantom. In their texts with Reed, Eidolon mentions ghosts often.
- Excluding Aria and Five, Reed and Mariya have evacuated 6 other people from the monster kingdom.