Aoi Tanio is a woman convicted of murder whose case brings Kotoko Iwanaga into the picture. She's the reason the Guardian Serpent of Tsukana Swamp reaches out for help in Chapter 15 of the manga and Episode 2 of the anime.
Tanio became the prime suspect after dumping a body into Tsukuna Swamp. What made her case particularly damning was the serpent's testimony—it claimed to have heard her say "I really hope they find it" right after she disposed of the body, which basically amounted to a supernatural witness statement.
Content compiled by AnimeList.moe from publicly available sources.
