She is loyal wife even after her divorce. She has sex with Masuo in the first place is because a girl tricked her into it. Later in order not to lose the only human connection she has, she let him sleep with her.
Hitomi Ito and Hitomi Onizuka (Mrs. Yamada) are both separate entity. She uses Hitomi Ito's and pretends to be a senior in Masuo's college to keep an eye out on him and live her college lover fantasy. She doesn't know Kunio that well.
On the other hand, the Hitomi Ito you are calling Kunio's girlfriend has yet to appear outside of that one cutscene.
Thank you for the creator's plot explanation. This is my last plot-related question. During today's gameplay, I noticed another issue. When the protagonist has sex with Hitomi Ito in the truck, he mentions that there is residual semen in her anus and speculates that she must have been with her boyfriend shortly before, which is why it's still there. What's going on with this dialogue? This should be Momoko Yamada pretending to be Hitomi Ito, right?
