Okay, re-reading this a couple more times I've revised my opinion. Hijiri didn't sleep with Kanako, but she sure as hell wanted to. The little omake drawing at the end about being unfaithful is about her desires, rather than something she actually did.
However, that said she does come off as an insincere player for most of this, especially scenes like this one, where she's known as a "lustful demon" and gives that "heh, easy pickings" look at the inexperienced Shinki. And later on, she thinks to herself that her intention was just to "mess around a bit" where Shinki is concerned. Hell, she doesn't come off as sincere at all until page 35, when she finally opens up about how she feels. It's like she didn't realize the depth of her own feelings until then, even during the whole time she was in Makai, which of course would explain why she had desire for a fling with Kanako (but Suwako was like NOPE!).
tl;dr Hijiri is terrible, but redeems herself a bit at the end.
last edited at Jun 16, 2015 4:11PM