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.

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 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.

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 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...

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.

The story about Caramel Theater, their owners and workers with different personalities! Love, friendship and family will tie them together or make them want to strangle each other!

A volume of cute and romantic short stories:

• Caramel Milk Tea
Nozomi, a high school student, did not believe in love at first sight. But it happens that she is in love with a cafe clerk. Immediately she declares love to him. But because he gives her the cold shoulder, she is disappointed in love. Now she begins to work part-time for the cafe where he is working.

• Futari no Jikan (Two People's Time)
Eri hangs out with her childhood friend, who she hasn't seen in four years. They've changed over the years, but certain feeling have stayed the same.

• Kirakira no Kimochi (A Sparkling Mood)
Narumi tries to make the most of her time before she moves on Christmas Day, but falls unexpectedly in love.

• No. 1 Girl
Saiko must choose between her career or her love.

Orange Kiss contains Happy Words, a prequel of No. 1 Girl.

Putting aside his feelings, when Sora found out that his childhood friend Rui got divorced, he decided to pay him a visit. After meeting for the first time in six months, Rui's overbearing way had not changed. Will he unreasonably decide to live together like they used to, ignorant of Sora's feelings!?
Also in this compilation: 19th Century England: On the stage, an aristocrat and a steward in a master and servant love relationship "Daryl".

1) Cherry
2) Cranberry
3) Dark Cherry
4) Daddy
5) Daryl
6) Daryl - Extra Story

In a faded little rural town, Kaworu lazily lives out his days. Until the day his destiny strikes, and he meets an angel. Just moments later, they quickly take off... Together, away from this reality!

1-3) Cherish
Chihiro is starting university where her deceased mother and the man who raised her attended. She wonders what's in store for her in this new chapter of her life, but is almost immediately confronted with her past! Her old boyfriend is back in the country and on top of that, who is the strange man watching her?

4-5) Happiness
Akatsu-kun is a young man in love with his work partner, Kayano-san, a pretty and intelligent woman. One day, while Akatsu-kun was shopping in a little store, he saw a little boy who was reading a Playboy magazine, then, Akatsu-kun realized this little boy is Kayano-san's son!!

From StarryHeaven:

High school first year Sara has a secret…she’s really the super-popular singer SALA! However, as an idol her gender is kept a secret from the world in order to sell her music. But can Sara win when she wants to be just Sara…?

Charisma is a multi-perspective story following several different characters. The central character is Archer, whose childhood was rather messed up due to the influence of his mentally unstable family members. His father Ed is afflicted with bipolar disorder, while older half-brother Murphy is autistic.

The story follows Archer as he works with Deano to infiltrate a drug cartel that has a long-standing enmity with his gang. Along the way he meets Ken, who's also infiltrating the cartel (for the police). What follows is a multitude of conflicts between different gangs, members of the same gang, and seemingly random people.

The chapters in the first volume are not in chronological order; the story begins with Archer being released from prison, then goes back to the past and how everything began.

NOTE: Cover image is from volume 3 of the 2001 edition.

After being the only survivor in a plane crash and losing his parents Ryouga, a young boy, is troubled by nightmares and taunted by other kids at his school. Fed up, he agrees to transfer to another school intrigued by a mysterious offer and the hope they may know why or how he was able to survive the crash. Arriving at the new school, he learns he must play a little card game with three others before being accepted... one card each, one vicious monster each, gruesome deaths for all save one.

From MangaArt:
Kisaragi Arisu has wanted to be an idol for a long time. When she blew her last chance, a guy ran into her and dropped his watch. She tries to find him, but finds herself in Channel W director's office instead. The director told her he needed a girl to do a report on his show. She readily took the job, thinking that this was a debut she was looking for. Will this be her big break?