Oiso is a minor character of the series Manyu Hiken-cho , appearing in episode 4, "Divers and Breasts". She owns a restaurant in the seaside village to Matsue and in the past was the best of alla divers, nicknamed " the Mermaid of Matsue" due to her incredible agility in the water.
Anyway, growing up her body became more curvy and her breasts started to hinder her movements in water to the point she had to stop diving. Oiso deeply regret not being able to dive again and despise her big breasts, a behaviour really unique in the universe of Manyu Hiken-cho, only shared by her fellow female divers. She is often teased by her fellow diver Mitsuki which have a slender body more suitable for the job.
During an octopus hunt, she is captured by a giant octups while saving Kaede . Chifusa dive to her rescue but the octopus use Oiso as a shield, forcing the swordswoman to unleash a "Breast Slash", sacrificing Oiso's breast but inflicting a deep wound to the octopus. Due to Chifusa inexperience in water, she cut the octopus but not enough to kill him for good. this time is Oiso that rescue her, displaying incredible speed and agility in water, dodging the octopus tentacles, stabbing it to death and taking Chifusa to surface.
Like the majority of characters in the show, Oiso is smitten by Chifusa beauty and offer her to remain in the village and to be Chifusa's "Onee-san", implying some romantic interest, much to the chagrin of Kaede that is quick in reclaiming her childhood friend.
Title: "Divers and Breasts" ("Ama to Oppai" - 海女とおっぱい )