The girl I love is my brother's ex-girlfriend. She's a trashy girl who's clingy, stubborn, and takes advantage of my weaknesses. But I can't seem to stop our relationship.

High school girl Akari mourns the loss of her mother who died when she shielded Akari from an oncoming truck a year ago. She discovers an adventure game her mother had gotten for her birthday some time ago, but never had a chance to finish. Suddenly, she is summoned to be the mother of the Demon Lord's 10 children in another world...

From MangaHelpers:
Hagoromo Maki is going to an all-girls high school, a special one that allows its students to commute by plane. She and her first friend at school, Hasegawa Motoko, have found a plane and fixed it up for themselves to fly! However, it turns out that it was the plane used by a legendary pilot who protected the airspace for their school, and their school's ace pilot does not approve of them having it. Will Maki be able to fly and protect the air for their school?

Arion, a young man, is kidnapped by Hades as a child, raised to believe that his mother was blinded by Zeus, and that killing the ruler of Mount Olympus will cure her.

From Chinatown KM:

Based on the 40mP million-view hit Vocaloid song Shiryoku Kensa, Miyashita Megumi, also known as "Gumi", was one of the only survivors in a plane crash thirteen years ago. Since then, she has been able to see two copies of every person and one of them always has a "C" on them. Thirteen years later, she now wears glasses, yet her left eye remains a puzzle. And then, her friend goes "missing".

Fuuko is a shy girl—so shy that she has trouble talking to strangers. Determined to change herself, she starts working as a waitress at Café Rhodos. The people she meets on and off the job will lead her through the often frightful world of adolescence, and beyond.

In the Kingdom of Bandiera, the royal prophet has announced the incoming resurrection of the Demon King, 500 years after the first King sealed it.

Four heroes were thus chosen and sent to the location of the Demon's prison. Meanwhile, Charlie Barett, a modest gate keeper, is stuck at the Royal City with a job he hates, spending his days complaining to his captain and his co-workers; one of which is the noticeably generic-looking Jimmy Monbarn.

A female author of lewd mangas frequently finds herself without motivation to work.
In those cases she calls over her married neighbour, Nanako-san.

The protagonists are Ryouga, a boy with superhuman strength, and Miruto, a girl who works for an organization tracking the villains. The plot centers around "Burst", the ability to use crystals with Pokémon stored in them to transform into human-Pokémon hybrids. Ryouga's Burst is Zekrom, and one of the villains' is Excadrill.
On rare occasions they actually use Pokémon.

Souya is a sullen salaryman whose days are spent chasing after work deadlines all while still keeping his dream of becoming a manga artist. To him, his one and only support in life is none other than his beloved wife, Yuuka. One day, on his way back home, he was suddenly caught in an accident and before he knew it, an alien had taken hold of his body! Not only that, the evil threat of another alien is closing in on Yuuka too!

Here comes Souya as "Otto-man", his superpowered husband form, as he jumps in to save his dearest wife! Have a taste of his love for wife and this world!

Appraiser Laurent was kicked out from his guild 『Golden Hawk』. It was a result of one-sided jealousy from the guild leader, Lucius. Feeling hurt and crushed by the sudden dismissal, a female mage named Lilianne gave him a helping hand. Together with her, he decided to create a new guild to mess with Lucius. Eventually, those talents that Laurent found through his appraisal skill will create the strongest guild.

"Newlyweds" Akira and Keiichi moved to a new place and are adjusting to their new lives. Akira works with kindergarteners while Keiichi is an editor.

Having reached retirement age, Hiraga Raizou (65) lands a new job that pays extremely well! And it’s top secret!! With no guarantee on your life!!! When he sets out to do the company’s mysterious work, he encounters a little girl at their “workplace” and gets pulled into an incident. Raizou took on the job to pay the medical bills for his grandson’s incurable disease, but what awaits him in this line of work…!? Author of “COPPELION” Inoue Tomonori brings to you an immoral crime action manga!! An elementary school girl and a tough old man team up to rampage through the underworld!!!

(From Fallen Angels)

16 year old Miyahara Momiji starts working part-time at an izakaya called Koharuya.

Hanadera Keisuke is obsessed with yuri, and he's read all the Yuri Hime manga. However, he is sadly aware that there is one thing a lesbian romance absolutely does not need-- him. One day, a student named Akane transferred to his class from an all-girls school. She used to go to school with another classmate, Saori, and Akane immediately tried to pick up their friendship. However, her clinginess, desire for alone-time, and intensity all convinced Keisuke that he was finally seeing yuri in reality! Soon enough, Keisuke had stalked them enough to see a love triangle between Saori, newcomer Akane, and their standoffish classmate Matsuoka. Is there truth to his observations, or is he viewing the world through yuri-tinted glasses?

Note: Comic Yuri Hime is not a seinen publication. It doesn't come under any of the four major demographics.

Everyone in a high school is suddenly transported to another world. As they are attacked by monsters they find that some of the students now have superpowers. They organize themselves into two groups. Those without powers are called the Stay Home Group and those with them are called the Exploration Group. On a day where most of the Exploration Group is out, the remaining ones that stay behind start the slaughtering and raping of the Stay Home Group.

Here we meet our protagonist, an average guy with no abilities who had just barely escaped death at the hands of his schoolmates. He ends up far away in a cave on the verge of death, exhausted and wounded when a slime enters the cave and starts eating away at his hand. All he can do is utter, “…please… someone out there… help me….”

Shikada Kokonotsu's father owns a rural sweets shop, and his plan is for Kokonotsu to take it over one day. However, Kokonotsu wants to be a manga author instead! One day in summer, the cute but weird girl Shidare Hotaru, from the famous sweets company, comes to pay a visit. Apparently, Kokonotsu's father is famous and she wants him to join her family's company. However, he will only agree if she can convince Kokonotsu to take over the family business!

A young girl named Rahzel is booted out of her house one day by her father with the instructions to 'see the world.' And so her journey begins. However, she won't be doing it alone... for she befriends a stoic young man named Alzeid seeking revenge for his father's murder.
Alzeid and Rahzel are like oil and water... but even still, they feel strangely drawn to one another. This could be either the result of an underlying attraction -- or their shared powers with magic. And so, the two reluctant allies travel from town to town, using their powers to help the helpless, while discovering their own respective places in the world.

Promoted at the speed of light, the star of JK Electronics, the ultimate career woman, yet always forever alone Cha Ju An!

Kind, honest, full of vitality, and with the looks and physique to rival a model, yet bumbling fool Yoo Do Jin!

With such different personalities, one day these two… #suddenly #withoutwarning #during #officehours switched souls?!!!

Due to this switch, their lives are made a mess!!
But what is the reason for this switch?
And that reason is to match these two up by force!

By the same author as What's Wrong with Secretary Kim.

Meet Kaichi Takemura, aged 82. Owner of the Takemura Dojo and master of the Churyu Akai-Jutsu martial arts, he is now wheelchair-ridden due to severe lung cancer. He is estimated to only have two months left to live.

Realizing his time on this earth is ending, Takemura creates a bucket list of things he wants to do before then, which he shares with his nurse Tokiko Tanaka. However, one day his illness flares up, and Tanaka wheels him to the nearest doctor as fast as she can.

When he comes to, he finds that the two of them aren’t in the emergency room of the hospital. In fact, it doesn’t seem like they’re in Japan at all…