Nui is a Mushishi and the narrator of the anime series. Her voice can be heard in many episodes. Nui's appearance is very similar to the series' protagonist Ginko. Due to her exposure to the Ginko Mushi, she has white hair and only her left eye (which has turned green). She is tall and usually wears a dark kimono. Since she doesn't have her right eye, she keeps it shut. Nui's appearance was different before being exposed to the Ginko, she had long, black hair, dark eyes, and wore a brighter kimono. Similar to Ginko, she smokes Mushi-Tobacco to keep Mushi away. Nui is a Mushishi from an isolated yet beautiful village in the mountains. Due to her tendency to attract Mushi she rarely stayed in her village and often used to travel from place to place driving Mushi away. She still often used to visit her parents, friends, and her husband and child. Due to her research on the Ginko, she was constantly exposed to its light and that changed her appearance. One day, she found an injured boy named Yoki in the woods who had lost his mother in a landslide. She tended to his wounds and explained to him the nature of Tokoyami.

Spoiler

One day while returning from her travels, she learned from other villagers that her husband, parents, child, and her friends had disappeared into the mountains. While searching for them she found out that there were Tokoyami living in a pond. Once she realized that her loved ones were turned into Tokoyami by the Ginko, she then decided to live near the pond and studied the Tokoyami and the Ginko that lived within it. She used the fish in the pond to conduct her research only to find out that eventually all the fish exposed to the Ginko turn into Tokoyami.

Spoiler

She willingly exposed herself to the light of the Ginko and turned into Tokoyami. Before completely fading away, Yoki grabbed her hand and was also consumed by the Tokoyami. Within the Tokoyami, Nui's spirit guided Yoki and told him that in order to escape, he would have to sacrifice his memories and one of his eyes to the Ginko. The Ginko then awoke and consumed the rest of Nui.

(Source: Mushishi Wiki, edited)