Karen Bee - Part 1

The five girls' problems with supernatural beings have been solved, and it's now summer break, with the culture fair over. Koyomi Araragi has two little sisters, self-proclaimed "heroes" nicknamed the Fire Sisters, Karen and Tsukihi. What "fake" beast does Karen confront? What supernatural creature of the holy grounds, whose power surpasses even a vampire, has Tsukihi absorbed?