Moon's Mysterious Sister is a manga-exclusive character.
She has a bald head with pigtails on it, and she wears normal schoolgirl uniform. She has anime eyes and a small nose. She seems to be as tall as Cony.
- The main 3 seems to follow the "Big brother, little sister" rule, as Brown and Moon both have younger sisters, and Cony has an elder brother.
- She seems to refer his brother Moon as the Prince of the Moon.
- Her only appearance is Chapter 25. After that, she was never shown again.
- Like Cony's brother, her name is unknown. The possible name for her would be Luna, the Spanish word for moon.
- She once attempted to repair his brother's spacecraft in order to depart for the Moon with him, but she gave up shortly after.