Masako Yoshii is the biological mother of Rin Kaga. She previously worked as a housekeeper for Souichi Kaga, Daikichi's grandfather, and the two became involved, leading to Rin's birth. At that time, Yoshii was an aspiring manga artist with a strong drive to establish herself in the industry. She chose to leave Rin in Souichi's care and did not publicly acknowledge her as her daughter, instead being known to Rin simply as a kind maid or aunt. Her primary motivation is her ambition to become a successful illustrator, which she prioritizes over her responsibilities as a mother. In the story, she appears as a supporting character who meets Daikichi to explain her past decisions and to suggest that Rin take Daikichi's surname to avoid social difficulties. Her personality is self-centered and highly sensitive, with a single-minded focus on her career that leads her to neglect personal obligations. She is not depicted as cruel, but rather as someone unable to provide the care and stability Rin requires. Her key relationship is with Rin, to whom she is a distant birth mother rather than an active parent. Her interaction with Daikichi is brief and largely practical. In the anime adaptation, Yoshii does not undergo significant personal development, remaining a figure from Rin's early past rather than an evolving presence. Her notable ability is her work as a manga artist, which she pursues with dedication and which eventually brings her professional success. This success, however, comes at the cost of her connection to her daughter, and she never fully reconciles her role as a mother with her career.