'S' is a known as a bully caretaker, who avenges her bullied clients -- for a price, of course.

However, in reality, 'S' is a nerd named Kang Song Ih, with a twin brother named Kang Ah Eul, whom she loves above anything. Ah Eul attends Chungha High School, a famous and expensive school, but over winter break, he attempts to commit suicide. Song Ih, determined to understand why her brother tried to kill himself, attends Chungha High School, pretending to be Ah Eul -- uncovering the reasons why Ah Eul was driven to suicide, including one mysteriously handsome but "bad" guy named Suh Ga Yoon.

Read as you follow her life while staying in an All-Boy's school and living in a dorm -- with a male roommate, who happens to be a famous singer and someone she as "S" had once nearly beat up.

Watch out! Whoever bothers "S"'s brother, prepare to face her!

Itsuki is the number one host at the club 'The Rose Garden'. Somehow, he has fallen in love with Ryou, the shy, innocent newcomer. Itsuki is unsure how to confess his feelings, but then, for the club's 5th anniversary, he is asked to wear a very special costume... Will Itsuki's beauty catch Ryou's eye?

From Seven Seas:

Being a girl is harder than it looks...
For Hazumu, this couldn't be more true, because just the other day, she...was a he.

Shunned by the girl of his dreams, Hazumu loses himself in the mountains and is promptly squashed by an oncoming space ship. The alien inside, feeling guilty, rebuilds Hazumu's body...but as the wrong gender!

Now Hazumu must learn how to be the girl his parents always wanted while dealing with the trials and tribulations of being caught in a love triangle between two girls--his childhood friend, Tomari, and Yasuna, the girl who rejected him but is now strangely attracted to him/her!

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?!

Kohamaru comes from a peculiar family - each member can switch bodies with his or her soul mate! His parents can switch bodies, and his aunt, oddly enough, can switch with her cat! Now it's Kohamaru's turn...

When Kohamaru literally runs into Nanao, the girl of his dreams, they switch bodies. True love, right? Well, unfortunately, she's in love with someone else. And if that weren't bad enough, the "someone else" turns out to be Kohamaru's twin brother Konatsu... who can reverse their transformation with his own kiss!

Keidai Saeki is a high school student who remembers his past life in Pompeii, living as the Italian warrior Sirix. His friends around him are all also reincarnated - although they don't quite remember things the way he does. Keidai's wife from the past? She's now a middle school boy - while his best male friend is now a cute girl?! Throw in a case of love at first sight, and you have a new look at romance - manga-style!

From CMX:

High school student Yukari Fujimaru knows a potential star when she sees one…because she sees the special "aura" they give off, in the form of angelic wings. Yukari's fellow student, the beautiful Ryoko Katsuragi, has a small pair of wings herself, and is one of these aspiring talents. She's so talented, in fact, that no one even knows that "she" is actually a boy in disguise. Now, to make her own down-to-earth dream come true, Yukari must help her new friend become a star.

From Impossibility:

Morisawa Kai and Sato Tatsuki are best friends. However, unlike Kai, Tatsuki is completely unreliable, lazy, and ill-tempered. One day Tatsuki takes Kai out driving in his new motocycle only to get into a bad accident that leaves Kai dead. However, Kai's spirit ends up in a body of a girl named Fujii Marika. After seeing how cold and unaffected Tatsuki as well as everyone else was after his death, Kai transfers to his previous school in which he starts noticing flaws to Tatsuki's cool mask...

1-5) Renai Jikaku Shoujou
The beautiful, strong Agatsuma Tamaki has a weakness~ his kohai* (*junior in age or rank), Sawamura Kei, who simply just drives Tamaki up the wall by his mere existence. On one fateful night, Tamaki’s guilty secret was discovered by no other than…! Will romance blossom between this wild but emotionally inept couple?

6) Kimi to Boku no Itsu Pobun
Horiuchi has always been precognitive and his cousin always gave him a hard time for that. Now his bully of a cousin has to stay with his family temporarily and his ribbing doesn't stop. Oneshot also found in Saikou no Kimi

7) Afterword

Quite possibly the best and most reknown fighter of all the schools is a red haired girl, solely referred to as the Red Lion. But as the Red Lion suddenly disappears one day, we find that's only the beginning to the trials of real "Red Lion".

From Viz:
Hime Kino's dream is to one day perform voice acting like her hero, Sakura Aoyama, from the Lovely Blazer anime. So getting accepted to the prestigious Hiragi Academy's voice actor department is the first step in the right direction! But on her first day of class she meets Sakura's grouchy son, Senri, who tells Hime that her voice is awful and Lovely Blazers is stupid. Hime will not let that stand unchallenged. She'll show everyone that she IS a voice-acting princess, whether they like it or not!!

Talentless, shy Tsubasa has just been accepted into the prestigious Seiko Academy. He soon gets mixed up with the most popular kids in school -- and the entertainment world! Forced to join the pop idol group RINX as their lead FEMALE singer, Tsubasa soon finds himself stuck in the middle of a cross-dressing comedy full of mistaken identities, blackmail, and kidnapping!

Midori Yamate is a 15 year-old girl and is very much a tomboy. When she was young, Midori meets a boy called Tsukasa Hino, who came to the island she lives on for a holiday. He teaches her to play soccer and she falls in love with the sport. Midori is inspired by Tsukasa and continues to play soccer in hopes that they would meet again someday. When they do meet again Tsukasa does something unforgivable. Determined to defeat Tsukasa on the soccer field Midori enrolls in an all boy high school.

Contains a short story of 2 chapters called Shounen x Cinderella in the 8th and 9th Volumes:
Sakuragi Azuma has a secret that no one should ever know about: he's actually Momo Kasuka, an up and coming female model! While shooting a new drama, he meets another arising newcomer named Yui Hazuki, and things are going to get heated!

[From JanimeS]:

Dohun Yu has been living in England ever since his family collapsed. One day, Dohun comes to Korea almost forcefully without proper preparation. Don't ask me why, but I think it's his personality. (He does do things without planning anyway). Dohun loses his bag as he arrives in Seoul (the capital city of Korea) along with all his money. He comes completely moneyless within a day. A week later, he encounters an ad where it said "Seeking a friend! Paying well...yada yada blah blah, girl only!". Still having his suitcase which was confused with his sister's, he had girl's clothes. Having no other choice, he applies.

NOTE: This series was abandoned by the author at 4 volumes, with the story incomplete.

In the play of life, all Hikaru Takabayashi wanted was to be part of the scenery. Tormented by jealous schoolmates ever since she appeared in a famous commercial in grade-school, Hikaru transfers to a prestigious all-girls' academy where she hopes to settle down to a quiet life of anonymity. But when her new roommate turns out to be the super-popular TV idol Izumi Kido, Hikaru finds herself thrown smack in the spotlight's glare once more! And as if living with a celebrity wasn’t bad enough, Hikaru discovers that there’s more to Izumi than her “good girl” image—she’s a he! And this is only the first of Izumi's many dangerous secrets! Now Hikaru’s the unwilling star player in a madcap tale of deception, abduction, humiliation and blackmail...

This is the story of a boy who one day walks into some strange wood and is saved by a nymphomaniac godess who can not only take over his body (turning it into hers in the process) but can also transform him into a girl.

From StarryHeaven:
Jun has always liked the super popular and hot Kazumi-kun. However, he always has girls competing for his attention. So when the newest issue of a shoujo magazine comes out, she buys it for the furoku (freebie): a paper charm that you write your wish on and put under your pillow so that it’ll come true. Her wish? To be with Kazumi-kun. She doesn’t expect the charm to work anyhow, as she’s never had any luck with them. To Jun’s surprise and delight, in the morning she wakes up to find that she’s friends with Kazumi–but in a boy’s body!

Sixteen-year-old tomboy Ito Miura has become fast friends with her schoolmate and fellow drama enthusiast Makoto Amano. The problem? Makoto's a boy who must pass as a female drama student in order for his family to approve of his acting ambitions.

From Tokyopop:

Guilt-Na-Zan is a vampire aristocrat who was sealed into a cross by Kyouji's ancestor more than 100 years ago. Now Kyouji has revived him--although his soul was resurrected into a cute female doll and can only transform into his real figure when he sucks blood from Kyouji's sister Tonae. But when you combine Goth clothes and hot vampires, slowly but surely everyone will start to feel like one big family!

The story of two best friends -- a girl who's convinced that she's a lesbian because of her "non-girly" tastes and a guy who believes he's gay because of his feminine personality, and their search for love.