Two twin brothers Ry and Key were trained in martial arts since childhood by their Martial Arts father, "Rokuu Riley" as they are now young adults a tournament begins in their city and they feel they are called to accomplish something much greater in life. And in their own ways they will not stop until they have succeeded no matter what challenges, threats or obstacles come into their lives.

Older brother forces younger into a sexual relationship after their parents' death.

From Infinity Studios:
Ever since we were children, we’ve heard stories about ghosts and about heaven and hell. Well, what if we told you another existence between heaven and hell exists? This is exactly what Suh Rin, an average high school girl, finds out one gloomy day.
On her way home from shopping around, Suh Rin meets with an unfortunate end when she is hit by an on coming truck. The next thing she knows, she’s being abducted by a stranger. Overwhelmed by the fact she had just been killed, Suh Rin has difficulty grasping the fact that she has entered a new existence in a realm of spirits. More importantly, she has yet to realize that tremendous supernatural powers lie within her… a power great enough to make her the target of abductions by tremendously powerful people within the spirit realm.
However, thanks to Young and the mysterious owner of a café called the Café Occult, Suh Rin is rescued from a new existence as someone’s servant. On the contrary, she joins Young on action packed thrills in which she assists Young fulfill requests as an ‘errand boy’ taking care of special requests by clients in a lawless world controlled only by power.

Listen to the world's various stories on Yoo Si Ae's radio show, Cactus and Radio.

Tomoe Tatsumi is a freshman attending university in Tokyo. Life couldn't be better: he's living in a really nice apartment for less than most people pay for a single room. This is all thanks to Mitsugu Kurokawa: salaryman, landlord, and (hopeful) lover.

It is due to Kurokawa's generosity that Tatsumi is able to live in Tokyo on such a tight budget. Kurokawa's intention is clear: to love, honor, and cherish Tatsumi until his dying days! And if big brother Souichi Tatsumi has his way - that day will come sooner than not!

For Kurokawa, progress can only be made by "challenging the impossible...." And thus, Takanaga sensei introduces us to her charming romantic comedy, Challengers!

- DramaQueen

The main couple from Koisuru Boukun also have a few short chapters through out the manga that take place before Koisuru Boukun starts.

A manga consisting of a 9 chapter story with 3 side stories.

Shinju no Kusari (Chain of Pearls)
The beautiful, genuine pearls we lust after belong in a chain... because that is how they bind our hearts. Hisa, the newest manager of her high school's soccer club, has a secret: a part-time job. If anyone knew, she would either have to quit the job she needs to survive or the sport that she loves. Sakyou, one of the team's most talented and anti-social members, finds out about Hisa's secret... so it's a good thing for Hisa that the handsome and high-handed Sakyou wants to keep her close to him! Or is it? (There is an additional one-shot of this story in Vol. 1 of LoveSupreme Principle)

A "Girlfriend" with Substance
(scanlated as The Fruit Called Grapefruit)
Minami Kanan's debut work. Sachi was so determined to become Takanori's girlfriend that there was no way he could say "no." Literally, no way. She's the girlfriend he was forced to have, and he treats her as such. They go out together, but it seems obvious that Takanori hates every moment of it. When Sachi finally has enough of his unaffected attitude, will it make Takanori happy to be alone again?

Unconditional Love Formula
Tachibana Kouya is a whiz kid, while Ayumi barely passed the entrance exams. They're at opposite ends of the spectrum, but Ayumi is determined to win Kouya's love by having him tutor her -- then, when she passes the physics exam, she's going to confess. Why would Kouya want to tutor such a poor student, though? Ayumi's love rival discovers that Ayumi had to bribe Kouya to do it, and so she feels no remorse when she ruthlessly foils our heroine's efforts to succeed in physics and love...!

The Rumored Natori-kun
High school freshman Ai just found out that the stranger she fell in love with at first sight is the notorious Natori-kun, who goes to her school and is rumored to ditch every girl he agrees to date. That doesn't discourage Ai, though, because he's been nothing but nice to her so far. What will it take to become Natori's girlfriend, or is Ai doomed to become just another rumor?

From Fallen Syndicate:

Seri, a somewhat directionally challenged girl, went to Umeda for a job interview. Finding herself completely lost, she stumbles across a slightly opened door and decides to try her luck.

She takes the stairs behind the door, emerging on a peculiar street half an hour later. Unknown to Seri, she has just mistakenly entered a place far more dangerous than she could ever imagine.

From Easy Going Scans:

Kei often gets involved in fights, despite of him being weak and always end up losing… One day, while helping a friend to bring some textbooks to an old building of his school, he and his childhood friend, Tomoe, are attacked by a monster. In order to save his friend, Kei will choose to become a Grave Protector!

* Cerberus, in ancient Greek mythology, was a hell hound that guarded the gates of the Underworld preventing those who have crossed the river Styx from ever escaping.

Ginta's father works at a cellphone company called "Choto Bank." Their new cellphone is a humanoid cellphone. Gina's father sent his son a prototype version of the humanoid cellphone, but Ginta's sister unexpectedly made some "changes" to the new cellphone and due to that it makes countless mistakes. The cellphone humanoid, named Momoko, slips on flat surfaces, rips Ginta's jacket, turns a jacket into a puzzle, and trips and nearly kills Ginta when a pair of scissors slip out of her hand. Ginta asks himself, "Will I live long in this condition?"
{Notice: This manga is different from traditional manga. Instead of reading right to left, it reads left to right.}

A collection of oneshots:

Her Carriage
Everyday Kouta comes to pick up Aya on his bike. Aya loves Kouta, but Kouta is looking for a girl friend. Will Aya ever tell Kouta how he feels?

Make-Believe Brothers
Art student Yuuta and Tomo are neighbours and childhood friends. But is that all they are?

After-School Strawberry Flavor
Everyday Tsuduru finds candy in his school desk; probably left there by a night school student. He starts to exchange letters with the unknown giver; but will they ever meet?

Finger
Although Akio, who has a hand fetish and confesses his love for Hayato, Hayato refuses to accept
the fact that he loves Akio as well, until Akio decided to leave to study abroad?! Will Hayato come to face his true feelings?

Four Leaf Clover
Yoshida unconsciously began noticing Yukimoto, a boy with weak health who always sits PE out. When Yoshida starts finding four leaf clovers in the class diary, he asks Yuki if it was him...

Cello Mellow
An attractive cellist by the name of Mahoro is a hopeless man who can't recognize love. Mahoro, who has never loved anyone before, was picked up by Sousuke on a rainy day one year ago. Woken every morning by Sousuke, the sleepy Mahoro finds himself drawn to Sousuke's smile.

Cecile wakes up one morning with a boy from her university beside her. Exactly how much did they drink last night? And now what are they going to do?

From DrMaster:
Supernatural phenomena and strange occurrences are no sweat for the Nanami Paranormal Investigation agency, headed by Asagi Nanami. As "freak" activity grows to a terrifying fever pitch, Asagi's assistants -- Naoki, Tokiko, and Mahime -- help to solve several major "freak" cases. Freaks are creatures with the ability to possess humans and prey on their weaknesses. However, Asagi and his gang are no ordinary humans; each is equipped with a special power to vanquish Freaks.

From Esthétique:
Campus comedies weren't always the leading plot of shoujo manga. The old shoujo manga used to focus on personal accomplishments, or becoming somebody. "Catch, Love" is about a girl who picks up a camera someday and realizes that she wants to be a photographer, like her father once was. She starts working for a magazine, meeting some celebrities and taking photos of them. But those old ladies chastise the girl because she falls short to emulating the great photographer, her father. She aims to perfect her skills and win compliments from those who loved her father's work... And of course, she receives a lot of support from her friends and the love of her life. A lonely heroine can't make a good story!

From Condensation:
The story centers around Keito Aoyama, a former famous child actor. Due to a childhood incident, she retired from her career early and withdrew from normal society. Now 16, she passes her days in boredom and without purpose. But one day she runs into a stranger who takes her to El Liston--a free school for high school students like herself, who don't have a place where they belong. Though hesitant at first, Keito decides to enter El Liston and subsequently, she begins a new stage in her life. With the help of her only remaining childhood friend, Taiyou, and her new classmates Rei, Momiji, and Kouichi, Keito slowly finds the courage to open up to others and to accept their support. Will Keito return to acting? Will she find true friends? Love?

From Entropy:

Jin Young is a good student with good grades, but she has a little bit of an unstable mind. Hye Jung is a cheerful girl who helps out at her mom's "snack shop", a sort of small restaurant that offers light meals to diners. Both attended the same school, and both transferred to an independent-study private school, where they meet Si Hyung who is an instructor there. Si Hyung used to live in the same neighborhood with Jin Young when they were young, but he's surprised how Jin Young has changed into a cold person.

Sequel to the novel River's End (Novel).

Yorozu lives in a love hotel. When a film crew comes to the hotel to make an AV, he has an encounter with Togame, the director. On the first day, Yorozu is asked to be a substitute actor and has his pants pulled down; it was the worst first impression ever! To make matters worse, he's worried about the openly gay Togame getting close to his younger brother, Satoru. Yorozu takes action and tries to keep Togame away from his brother, but...

Taken from "The Story So Far" part:
Anna Molmoque has become a doll meister apprentice (Dolce), something that she's admired ever since she was small. Excited, she was taken in by Dollhouse Casseiopeia, but the place is filled with people who are a little strange.

Looked after by the manager, Elsa Vuleria and the co-manager, Olga Black, Anna witnessed the two kissing! Olga was just waking up the sleeping Elsa with a kiss, but that was the first time Anna saw two girls kissing, so she was surprised!

A gripping reinterpretation/adaptation of The Illiad by Homer that revolves around Priam's daughter, Cassandra, who was believed to have been given the gift of prophecy by Apollo when he proposed to her.

By anonymous:
Nohae has been crushing on Dona, his neighbor, since they were younger. Dona is a girl who was obsessed with money as a child and would make her childhood friend Nohae actually pay her. Time has passed and now she's a teen, broke and living on the streets with her dad. Her dad meets Nohae and arranges for Dona to live with him. But Dona's not your typical heroine. She's one of those fake people, maintaining a perfect image at school but in front of her parents she's self-absorbed, a glutton, and is quick to nab a meal or swindle money. She's quick to anger and has a grudge against Nohae, but he likes her so much he's completely unaware.

In the Carramer no Yumemonogatari series (カラメールの夢物語シリーズ), also called "The Carramer Legacy" series: 1) Princess e no Tabi (プリンセスへの旅; Journey to the Princess) / Crowns and a Cradle When Prince Josquin de Marigny swept struggling single mom Sarah McInnes into a limo, he revealed a secret. Sarah's father had been in line for the Valmont throne -- and now her son was the heir! The brooding, blue-eyed prince promised Sarah everything she'd ever dreamed of if she would remain in Carramer -- except his heart.... Baffled and bewildered by the sense of homecoming, Sarah was torn. Josquin had given her a new family, but he'd also investigated her past, lied to bring her to Carramer and he'd be regent for her son. So once he took her in his arms, could she trust the promises he made? 2) Koi wo Wasureta Prince (恋を忘れたプリンス; The Prince Who Forgot His Love) / The Baron & the Bodyguard Watching over him... Playing royal bodyguard to Mathiaz de Marigny, the seductive Baron Montravel, was a temptation few women could resist. But the moment he was out of harm's way, Jacinta Newnham had fled his château -- and abandoned memories of moonlit nights that left them both weak with unfulfilled passion. Now stricken with amnesia and in danger once again, Mathiaz summoned Jacinta back to his side. Jacinta vowed to protect his life, but the greater peril was to her heart -- and the haunting secret she could never reveal. When the truth surrounding Mathiaz's accident -- and Jacinta's connection to it -- surfaced, would he force her to leave, or refuse to let her go again...? 3) Prince no Shukumei (プリンスの宿命; Fate of the Prince) / The Marquis and the Mother-To-Be Back in her lush, beloved Carramer, Carissa Day purchased the perfect B and B for raising her babies-to-be. Trouble was, an aristocratic "intruder" proved she'd been swindled -- her new home was actually his royal lodge. Worse, he proved to be Carissa's teenage crush, Eduard de Marigny, Marquis of Merrisand -- now more irresistible than ever. Penniless and pregnant -- with triplets! -- Carissa had to flee. Eduard, however, had other ideas. Namely, a partnership giving Carissa a title and protection -- and giving him an instant heir. Still, even with reignited passion burning between them, even with a kingdom at her feet, could Carissa wed her first and only love -- and forever forsake having his heart? Descriptions taken from FictionDB. Note: These are #4 - #6 of Valerie Parv's The Carramer Legacy series.