Kamishima Kyou and Kamishima Rei are twin siblings, whose parents divorced after they were born.
Kyou went to live with their father, while Rei lived with their mother, neither sibling is aware of each other's existence.
Though recently their mother died and so Rei ended up moving in with them.
Kyou is egotistical and has a huge inferiority complex. He used to pride himself of his excellent grades at school. But he was no match for Rei, who often pipped him in the academic rankings.
Annoyed by the sudden appearance of Rei usurping his top spot in school, he often takes out his anger at Rei by raping her all the time.
He doesn't care if Rei gets pregnant, as that'll mean she would be disgraced and would no longer be seen as an honours student, thus ensuring he stays top.
(Me: Yeah, how that does work once people find out you were the one raping her? Anyway, let's not get in the way of hentai logic here)
P25-26 is a bit confusing, as Rei triumphantly smiles and says her body is too young to get pregnant, so Kyou's plan won't work. And she knew her brother is actually enjoying having sex with her despite his grumblings.
(Me: Again WTF? Character data in P27 says she's already third year in junior high, so she's old enough to be able to get pregnant. Would have been less confusing if she says she is diagnosed to have an infertile body)
So they're basically like Matou Shinji and Matou Sakura from Fate Stay Night. Shinji thought he dominates Sakura by raping her all the time, but little does he know that Sakura only LETS him because she pities how pathetic Shinji is.
Base +7, caesarsama +6, ScrawnyJohny +6, hoigoigoi +24