Tsuneko, a bar hostess, becomes entangled with protagonist Yozo Oba amid his severe personal turmoil. Having fled a prior relationship due to domestic violence, she establishes a complex yet swiftly destructive bond with Yozo. She introduces him to painkiller abuse, fueling his dependency and addiction. Financially motivated, she depletes Yozo's dwindling resources to sustain herself. Concurrently, she engages with a married pharmacist, illegally sourcing drugs through this affair. This toxic triangle enables her to manipulate both men for material gain and narcotics access.

Her influence accelerates Yozo’s moral and psychological deterioration, directly contributing to his financial collapse and isolation. Their relationship culminates in a failed suicide pact: Tsuneko dies while Yozo survives. Her death proves pivotal within Yozo’s narrative, amplifying his guilt and cementing his self-perception as irredeemable. Tsuneko functions as the catalyst for his irreversible descent into despair.