Makiyama Rihoko

A self-proclaimed beauty who is a friendly but mysterious nurse.
Using a special formula she obtained though a special process, she created a drug that make woman squirt breast-milk after being raped by a man who takes the drug.
She experiments with the protagonist, whose interested in the pill in question.
In exchange for being her guinea-pig, she aids the protagonist in his quest to train his female teachers.
With her carefree and unpredictable attitude, and her skillful sexual skills, the main character is often at her mercy.