The protagonist's loving big sister, perhaps a bit TOO loving at times!
Her position as sister is put to the test due to Yui's arrival, and a fierce rivalry ensues.

A tradition of Kotomusubi states that all shrine maidens should be cross-dressing males in order to not anger the Goddess, Mishiro is one of such maidens.
For Mishiro, marrying the protagonist is as certain as shooting fish in a barrel.