An incredibly busty female biology expert, so skilled that when she was hired on to the research team, they made sure to staff the team with only females just for her, and is also the college's nurse. When she was a college student, Yayoi was cursed with a fear of men (where in the manga this fear seemed to originate from seeing her parents having sex) so great that she would go into panic attacks whenever she even saw a man, though showed signs to have a deep desire to be a servant. So she was curious that her classmate, Mei Momozona, took an intimate relationship to a student she was tutoring, Sorata Muon. But later, when a valuable sample was targeted by Mouse to be stolen, Yayoi found herself trapped when one of her panic attacks triggered a major fire in the lab she was in. It was Sorata, who was really Mouse, who had rescued her and allowed her to come out of her shell (Steals 10 to 12). She is one of his most devoted servants, and is always trying to make him some concoction to give him more sexual energy.
She has shown a strong desire for him to walk her around naked in public like a dog, and even having sex in public.