Inukai Tomomi

She is actually the protagonist's sister, their mother acted as a surrogate for her older sister who is infertile and thus Tomomi was raised by their aunt as her own child.

The cause of the spring water's pollution was due to a blockage caused by natural disaster, after the problem was solved with protagonist's help, she decided to conduct an experiment of spring's water with fertility by purposely make some of the young female resident of the village pregnant. The test subject was three maidens from the village and herself.