Yoarashi Okinu, born Harada Kinu around 1845, had a background shrouded in mystery with various accounts suggesting she was either the daughter of a samurai from Awa Province, born in Edo, or the daughter of a fisherman from Jōgashima on the Miura Peninsula. Due to financial hardships, she reportedly sold herself into the geisha profession, adopting the name Kamakura Koharu. Her beauty and charm garnered significant attention, leading to her becoming the mistress of Ōkubo Tadayori, the daimyō of the Karasuyama Domain. Together, they had a son. However, during the Meiji Restoration, Ōkubo distanced himself from her, leaving Okinu to navigate life independently.

Following her separation from Ōkubo, Okinu became involved with Kobayashi Kinpei, a moneylender. During this relationship, she encountered Arashi Rikaku III, a kabuki actor. The two began an affair, and Okinu became pregnant with Rikaku's child. Desiring to be with Rikaku, Okinu conspired to eliminate Kinpei. On March 2, 1871, she poisoned Kinpei using arsenic. After the murder, Rikaku harbored Okinu, but their involvement was soon discovered, leading to their arrests.

Okinu was convicted of murder and sentenced to death. Due to her pregnancy, her execution was delayed until after the birth of her child. On March 28, 1872, she was executed by decapitation at Tokyo's Kozukappara execution grounds, and her head was displayed publicly for several days as a warning to others. Rikaku's death sentence was commuted to three years in prison, after which he resumed his acting career under the name Ichikawa Gonjūrō II.

Okinu's case became sensationalized in the media, contributing to the "poison women" (dokufu) narratives prevalent during the early Meiji period. These stories often blurred the lines between fact and fiction, portraying women like Okinu as femme fatales whose beauty and cunning led them to commit heinous crimes.