Kei is a normal--if very pretty--young boy in high school. One day, he falls ill and is taken to the hospital. They find, much to everyone's surprise, that "he" actually has a female genotype even though "his" phenotype is male!! A little bit of surgical work solves the problem, and cool male hero Kei becomes innocent female heroine Kei. And what a female! "He" was cute before, but she is a beauty now.

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Kei Yoshikawa, a normal - if very pretty - boy in high school is one day informed that he is genetically female. The shocking relevation causes Kei to restart his life as a beautiful girl named Megumi, while keeping it all a secret from her very handsome male friends. And of course they immediately find out, but all begin hitting on her?!

From DMP: Why is Toru Kouno receiving such an overly warm welcome at his new all boy's school? He has yet to discover the secret system called "Hime (Princess)" in effect at the school. Boys who are chosen have to dress up as a girl in every school event! Little does Kouno know that he would be the chosen one... -- Contains the story Tore x Puri (Train x Princess) in volume three.

From Dragon Voice Project:
Family Complex... This is exactly what Akira Sakamoto suffers from, being the second eldest son of the Sakamoto family and the only NORMAL person. The rest of the family consists of beauties, from the parents to the eldest son and the 2 daughters. Poor Akira feels that he's totally out of place, and tries to hide this from the others... but then one day it all comes out. How will his family react to this? In this hilarious manga, every chapter is dedicated to one family member, starting with Akira. Family Complex isn't quite like anything else.

The second eldest son, Akira. Appears in another manga Princess Princess as a minor character.

From Easy Going Scans:

Sui and Sou are a pair of twins who are starting high school. Though raised by occultist parents, they were supposed to have a very normal high school life. However, once they arrived at campus they not only encountered werewolves but also vampires and even sorcerers! As more strange people appear one by one, will the twins' secret be uncovered?