This is the second entry in a series of yandere trap one-shots Ayune Araragi is doing for Mahalo. These one-shots don't share any common characters, they are only related by theme. The series is as follows;

Mamori Kusunoki's childhood friend Wakaba Kisaki has been dressing and acting as a girl for years, stemming from a promise to get married when they were kids.
(Source: MU)