Mokugyo serves as one of two central protagonists in the narrative, a fish-like being adorned with thriving plant life upon its back—a probable biological adaptation to its surroundings. Existing underground 10,000 years after a global tsunami annihilated an ancient civilization sustained by Mana energy from giants called Luna, this entity questions its existence alongside the talkative Toriiruka. Expressions of confusion and displacement punctuate their dialogue, including reflections like "Why are we here?" and "I thought we were in somewhere peaceful..."

Having survived the catastrophe that forced life beneath the earth, Mokugyo pursues truths about the ancient disaster and the potential to return to the surface world. This mission originates with Toriiruka and later unites them with Harry, a solitary needle-covered animal, and Yona, a descendant of the old civilization tasked with its rebirth. Mokugyo's role centers on survival, investigating the past, and the collective journey toward the surface alongside companions.