Io (いお) is a peculiar and shy girl with green hair who appears in Episode 5.
Prior to living in the swamp, Io was told that she was to be sacrificed by her village, becoming the bride of the swamp's spirit, in order to prevent a flood. She managed to save her village and took up residence in the swamp, and she would eventually become engulfed by the Suiko Mushi, transforming her hair to an unnatural hue of green.
Years later, Ginko was traveling through the mountains, noticing the peculiar on-and-off appearances of the swamp as he went along. When the swamp reappeared, he discovered Io standing in the water. Ginko spoke to her, and found out a little bit about her past and how the swamp saved her life.
Ginko and Adashino looked at maps from Io's time period and saw that the old traveling swamps lead to the sea, as if to die. Realizing Io's life to be at risk, Ginko made an arrangement to save her. Though the plan seemed to have failed at first, some fishermen discovered her body caught in one of the fishing nets the next morning. However, she was transparent and unconscious.
Io reverted back to normal, managing to survive, and began a new life in a nearby village, sad that the swamp had died, but happy at the same time. The villagers welcomed her with loving arms, as the swamp's death had provided the village with a prosperous bounty of fish to eat.
- Her name is learned only later, when Adashino addresses her, after her hair is back to normal.