"Kakumei no Hi" ("The Day of Revolution") by Mikiyo Tsuda 2 volumes
Kei Yoshikawa founds out that he is actually a girl only after 15 years has passed. Gender benders in anime/ manga usually entails magical or physical change. In this case there is no cross-dressing or hocus-pocus involved, blame it on genetics. I don't want to divulge any further than that since it's only two volumes short. And for it's shortness I felt like the character's motivation is weak. I had mixed emotions relating to the character in the confusion of his/her supposed gender. Nevertheless it was fun.