Akira and his twin sister Izuna are like night and day–Akira is tall, Izuna is short; Akira is passive and hesitant, Izuna is brave and assertive. On the outside, they don’t seem to get along very well, but they are secretly engaged in a sexual relationship. Things get shaken up when Akira receives an anonymous love letter. Story aside, Izuna has unusually colored hair.
Chapter 5 from Maban’s tankoubon “Hatsujou Lovers” (発情ラヴァーズ).