It took some doing, but I finally tracked down the remaining two Demon Circles, which let me summon Venus. Unfortunately for me, she was a good distance from where I found the last circle, so it was a bit of a trek to her enraged form. Good thing I have the spell that reduces the number of random encounters (from very high to merely high, it feels like).
Her design is a bit racy, so I'll put it after the cut.