Nau began as a water Land God corrupted into a Mad God by the Tribe of Void, forced to assault Tsukiko's village for the Jewel of the Thousand Wonders. Defeated by Senya and Hyoudou Shinsuke, Nau broke free from mind control. Its spirit then manifested as a small, amnesiac creature with no memory of its past. This small form attached itself to Senya and Tsukiko, joining their travels.

Senya first encountered it as a shadowy, animal-like entity; it shared rice balls with him and started following. Tsukiko named it "Nau" because it habitually ended sentences with "nau." Unaware of its past identity, Nau showed a strong affinity for water. Drinking large quantities temporarily amplified its spiritual power, triggering a physical transformation. In this enhanced state, Nau grew much larger, gained flight, and could carry Senya and Tsukiko.

Nau wielded water manipulation and elemental intangibility (limited to its head in the Water God form). Flight was possible only after transformation. Its original Mad God form possessed Large Size and tentacle-based body control, while the manifestation form was Small Size and required water absorption for temporary power increases.

As the journey continued, Nau eventually recovered its memories. It revealed itself as a separated spirit fragment of the local land god who became the Mad God Senya defeated. The shock of that defeat caused its amnesia. Following this, Nau used spiritual water from the King of All Things to heal its domain's withered marsh. This act restored its position as land god, leading Nau to depart from Senya and Tsukiko to resume its duties.