In Heian era, a noble chief councillor of states had 2 children who wished they were opposite gender. Lady Kira is strong, willful girl, while Lord Kira is weak, beautiful boy. Because of their characters, their father raised Lady Kira as a boy and Lord Kira as a girl.
 
The manga is based on a novel from late Heian era called “Torikaebaya Monogatari”.

Manga adaptation of late Heian novel Torikaebaya Monogatari (&#12392;&#12426;&#12363;&#12408;&#12400;&#12420;&#29289;&#35486;), literally "If only I could exchange them!". It is the tale of two siblings whose mannerisms are those of the opposite sex, and their relationships in the Emperor's court. 
source: wikipedia

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