I Can't Make It

Yatogame's home ec class is having a cooking class where they'll make dishes to potentially be served at the school's cultural festival. Yatogame is paired up with Jin's sister, Toshika, and Tadakusa suggests they make "oni manju." What does the "oni" part mean? That you suffer like a demon if you eat one? When Toshika eats one of the finished oni manju... Also, what happens when the two of them figure out who the other really is?! And Nagoya's "tamasen" are made in Mikawa?