Yoshino Tayū (1606–1643) was a tayū, the highest ranking courtesan (see oiran) and most famous female entertainer in Kyoto. Because she bears a striking resemblance to a woman called Ukiyo, whom Musashi once loved, Yoshino gets Musashi's attention from the very moment
"Ageha is the top girl, or "Tayuu," of the most popular, top-rate yuujo brothel in the pleasure district of Gion in Kyoto.She is the sister of Ukiyo 浮世, a girl Musashi ended up living with in an attempt to protect her in order to fulfill a promise to his comrade-at-arms, Majima Gorohachi 真島五六八. In the end, Musashi was unable to protect Ukiyo, but left her with renewed spirit as a swordsman--with resolve to use his sword to protect those weaker than him.
Upon reaching Gion, Musashi spies Ageha walking about and immediately sees her resemblance to Ukiyo. However, he fails to broach the subject. When he first visits her brothel, the Tsuruya, to see her, she cautions him that all yuujo have a secret past they don't talk about. This is the stereotypical image of the sex worker in old Japan. Yuujo and geisha were not normally looked down upon. Rather, they were generally seen as someone with a very sad life. Normally, a woman would be used to marry a wealthy member of society or something like this, but, many women of the day were sold to brothels as part of their duty to their parents. So, they were seen as heroines who gave up their very bodies for the family (which was a very noble aim in those days).
Ageha conspires with the clandestine forces at work to change the government. She sets fire to the Tsuruya, allowing the assassination of an official, who came to take Haruka 遥 from Gion. At first, she offers the auspicious intentions of wanting to keep Kiryuu in town so that she could one day reap vengeance upon the man who (as rumor had it) had killed her sister, Ukiyo. Later, she admits it was really because she had feelings for Kiryuu and could not let him leave Gion 祇園.
Her assumption of a new name (Yoshino) has nothing to do with the political or clandestine plots afoot in the game. It was simply the practice of yuujo and geisha to adopt fake names. Obviously, this is for their protection as well as to remove them from the "real" world and make them seem more like a fantasy.