Hiroko Yamada is one of the three people living inside Guu's stomach world, first appearing in episode 8 of the TV series. A 22-year-old woman, Hiroko attempted to end her life by throwing herself into the ocean, only to be rescued unintentionally by Guu, who swallowed her while fishing. Despite bringing in a massive haul of sea life that day, Guu technically did not catch a single fish. Unlike Seiichi and Tomoyo, Hiroko is mentally unstable and struggles with bipolar disorder, making her volatile and unpredictable. Her suicide attempt was part of an elaborate plan to take revenge on her unfaithful husband: she believed that by dying, going to hell, and returning as a vengeful spirit, she could haunt him for the rest of his life. Now residing permanently within Guu's stomach world, Hiroko spends most of her time drinking alone atop a hill—a place the other inhabitants have learned to avoid. During a brief period when Guu swallowed Yumi-sensei, Hiroko unexpectedly formed a fast friendship with her.