He Took Mine

Yuna gets stuck babysitting Ma-kun, the spoiled kid of a rich family, for the summer, and the brat turns out to have a libido that outpaces his manners. Kazuhisa, who's had a crush on Yuna since forever, has to watch this play out from the sidelines while his classmate-slash-coworker fends off a child with the sex drive of an adult and none of the restraint.

The setup is simple: an inexperienced woman, a spoiled boy who won't take no for an answer, and a summer that keeps going sideways. Whatever line Yuna thinks she can hold, Ma-kun finds a way around it.

You move through the sandbox as Kazuhisa, poking around the house and town while the situation with Yuna and Ma-kun develops on its own track. Corruption builds gradually, there are multiple endings depending on how things shake out, and the game doesn't pretend the protagonist has much control over what's happening to the woman he likes.