Horou (帆楼, Horō)? is the God of Doubt (and Trust), she was the first and therefore oldest conscious being in the world, and the self-declared representative of the Old Deus. Horou is a name given to her by Sora and Shiro; She had no name prior. It comes from the word "hollow". Until the events in Volume 7 and 8, she had existed inside of Miko as conditions upon winning a game they played when Miko was a child. She doubted everything, even her own existence prior to losing to Sora and Shiro. She still asks many questions to Sora.
Doubt and trust, the seemingly conflicting concept which Horou represents, might originated from the story of the tree of good and evil, where humans are sinned with ability to identifies good and evil, which marks them sentiments and capable of doubt and trust.