Yotogi no Kuni no Gekkou Hime

There was a beautiful princess called Arue in a certain small country. However, there was another girl named Selene, the second princess, another beautiful princess hidden in this country named Selene. Treated as an abominable child by her androgyny, kept secret by country, and lived a quiet life in a dark room. But Selene had a bigger secret than anyone knew. The inside of Selene was an old man.

(Source: Terminus Translation)