She aids him in his plans to take revenge on Aki Adagaki stating that it would bring a mutual benefit to them both, as it would fulfill his wishes and claims it might humble her master making her life easier. It is revealed that it had not actually been Aki that had named Masamune "piggy" but instead Yoshino who drove Masamune away, who did so because of jealousy and anger towards her mistress and hate towards the friendship that Aki and Masamune had. As the story progresses, it is revealed that Yoshino actually has had a crush on Masamune, however, Masamune turns her down because of his feelings for Aki.
Aki is Yoshino's mistress, the two share a master-servant relationship for a majority of the series. Yoshino performs multitudes of tasks for Aki such as getting her food whenever she is hungry, it is implied that Yoshino does everything for Aki as it is stated that Aki has no real knowledge of the "outside world" as shown when she takes Yoshino's advice to wear a cosplay dress to her and Masamune's first "date". Yoshino for most of her life despised her mistress for her cruel treatment of her which is shown when it is revealed it was Yoshino, not Aki, who drove away Masamune when they were children because she was angry at their friendship and wanted to see them suffer.