Yoshie is mentioned in Episode Three as the friend who tells Rin the story in 1945, a memory triggered by Yuria, the girl who dies after saying "The flower of death! It's going to bloom in Tokyo!"
Originally, Yoshie worked at a medical facility on Hagashima Island during World War II where she worked treating natives on the island of a deadly disease known as 'Higan', or the Equinox Disease, which caused horrible delusions and pain in those suffering from it. It was also theorized that the disease was also researched by the infamous 'Unit 321' of the Japanese Army as a possible biological weapon during the war. But all their research failed because of the non-contagious nature of the disease. All through Yoshie's time at the facility during the war, very few people ever caught the disease, and of those, only one had ever died.
After the war, the population had been forcibly removed from Hagashima Island, which was later reported to have been destroyed by geological events. In truth, the island's existence was removed from the world's atlases in order for the United States' government to use the island as a safe site for its own biological warfare experiments. Yoshie, in the meantime, had kept some of the records about the experiments and the disease and tried to sell them on the black market. She was nearly beaten by an angry mob until Rin stopped them.
In 2011, when a rash of bombings connected to the Equinox Disease occurred in Japan, Rin used Mimi's help in locating Yoshie, because one of the bombings had a mysterious tie to the death of one Yuria Takimoto, who had suffered from that same disease before falling to her death while warning Rin. By this time, Yoshie was old and near death, and was amazed to see Rin again. Rin lied by saying that she was actually Rin's granddaughter and that the Rin she knew had died years before. At first hesitant, Yoshie gave Rin the information she still had.