Hanako's father died during World War II on an island. Because of his strong feelings, the Yōka spread their seeds and scattered them. The flowers managed to ride the winds, growing from island to island until it managed to reach Hanako's apartment in Japan.
Every year in Hanako's apartment, the Yōka in the flowerpots bloom, overtaking the clothesline and creeping into Hanako's room. She had researched them and discovered that they only blossom in the southern jungles and shouldn't be able to bloom in colder places like Japan. Hanako gets help from Kitarō in finding the source of these flowers, finding more on each island they come across.
They find the source of the flowers on an island that has remnants of the war that happened a long time ago. They discover a large tree, finding the corpse of a soldier buried under, where Hanako recognizes the ring as that of her father's. She cries as she realizes her father sent the flowers to watch over her. With this, all the Yōka scattered their petals, dyeing the place red. Afterwards they lay her father to rest and depart from the island.
The Yōka appears in episode #20 of the sixth anime adaptation, Memories of the Yōka. After Soujiro died on an island of New Guinea, his feelings had imbued into the letter he was holding, this caused a field of Yōka to sprout around the sacred tree where his remains and those of his fellow Japanese soldiers were buried. The Yōka managed to ride the winds to grow more flowers, until they reached Toshiko's home, blooming around Obon.
When Mana and her mother are tasked by Toshiko to watch the flowers, Mana sends pictures of these flowers to Neko-Musume who shows it to Kitarō and his father. Upon seeing them, they visit Mana, which they inform her of the plants and how they work. Kitarō senses no malicious presence and wonders about the Yōka's origin. Mana is entranced by the flowers and decides to join them when they plan to find it. They find numerous islands with Yōka planted near their shores, but cannot find the source, until they reach an island past the equator, where Kitarō is sure of it. After venturing through the jungle they find a Japanese logging company and asks the foreman if he has seen the flower, he becomes terrified and Nezumi-Otoko tricks him into explaining. The foreman tells them that since finding the flowers in the jungle, horrifying noises are heard every night. During the night, the noises occur as Mana wanders through the jungle, eventually being lead by footprints to the sacred tree surrounded by fields of Yōka. Immediately, Kitarō and the others arrive and they discover the remains of the Japanese soldiers and are told by the foreman that he and his men were ordered to remove them. This intimidated the Tubuan who had guarded them, causing the nightly noises.
Kitarō finds the letter and gives it to Mana, Medama-Oyaji wonders how the letter managed to not rot away, finding it strange. He guesses that the power from Soujiro's feelings had took root in it and believes that because he was unable to tell his love what happened, is what lead to Yōka being born and rode the winds to Japan, making that year's blooming to be a final message, sensing that the place would be lost. Mana gives the letter to Toshiko once returning to Japan. After reading the letter, Toshiko sheds a tear and understands that her love did come back. The winds blow as silhouettes of Soujiro and a younger Toshiko appear standing among the Yōka.
Powers and Abilities
Accelerated Growth: The Yōka are able to grow very quickly, being able to wrap a room's walls overnight. The flowers are able to grow back after all of their petals are plucked.
Enhanced Endurance: Unlike normal flowers from warm jungles, the Yōka are resilient to the colder climate of Japan.