There are several CGs for the (intimate) scenes.
And no, I don't think it's possible to have a monogamous relationship with a character...
But I think something a little bit like a serious relationship would be with Bernard :3
The story is really good, the first time I downloaded this VN I was just looking for something "hot" lol....
But the story really got me xd
Also, don't download it yet, I think that tomorrow or maybe in a few days the new update will be released. The updates are monthly, at the end of each month, however, Hyao took the month of August off, but he said he would update the novel at the end of September, so, if everything goes well, it should be today, tomorrow or in a few days.