The Sickly Emperor Is Only Immune to Me - Chapter 291 - 288 Playful Banter

It is an era of artificial superintelligences, interstellar travel, gene medicine, and virtual reality worlds. Far into Earth’s future, 15-year-old Gu Ding is a student of Cerulean Military Academy by day, and tavern waiter by night. His ambition: Become the space pirate with the highest bounty offered by the Universal Federation Government. With the help of Neptune, a neonate super-intelligence, Gu Ding finds himself thrust into a galaxy of shadowy organizations, political underdealings, and otherworldly forces that lie beyond human comprehension…

The 7-year-old little crybaby met 11-year-old big brother who ran away from home.
[This man became the protector of redemption in his life]

The 16-year-old little dragon fell in love with a 20-year-old big brother.
[It’s a pity that the big brother didn’t recognize him as the little crybaby]

The 23-year-old single father and 27-year-old chief executive meets again.
[This time the little crybaby and his big brother are finally together]

Fortunately we met through hundreds of millions of people, Thank God, for letting us break through many obstacles and get back together again, falling in love with you and being with you again is the greatest fortune in my life.

- Description from Novelupdates

In a world where magic and divine power collide, the youngest son of Lucifer, a demon boy born with unimaginable strength, was chained and sealed away by his father to prevent him from destroying Hell itself. At the age of eight, his power became so uncontrollable that even the King of Hell feared his existence. Desperate to keep the demon's destructive potential contained, Lucifer made a pact with the great Merlin, who trapped the boy in the human realm, binding his powers with enchanted chains.Years later, with Merlin dead and a kingdom still fearful of the demon’s release, an elite group of knights, blessed by the gods, take it upon themselves to guard his prison. However, when a war between the mages and knights erupts, chaos provides the perfect opportunity for the young demon to break free.Hidden among the knights, disguised within their ranks, the demon plots his next move. But his escape is just the beginning. Hunted by both the mages and the knights, he must navigate a world where ancient gods, wizards, and warriors clash. As he travels deep into enemy territory, sneaking into the heart of the kingdom, he is driven by one purpose: freedom.Now, with his powers still sealed but his physical strength unmatched, the young demon embarks on a journey that will shake the foundation of both realms. Can the knights stop him before he regains his full strength, or will the demon's wrath bring both Heaven and Hell to their knees?As the demon hides in the shadows of the human world, a new chapter in the war between gods, magic, and demonic power begins.

15 years ago the world discovered magic as well as mythical beings (basically beings from various legends) in another dimension. Humankind learned to summon these dangerous mythical beings and use their power to great effect. Seven countries formed contracts with these “mythical beings” and gained their allegiance as a result. The mythical beings will choose their summoners on their 14th birthday and give them a stigma. Schools have been set up to train summoners and magical swordsman (basically bodyguards that give the summoners time to cast their spell) as a result.

15 years later after the finding of magic Kazuki Hayashizaki is chosen as a summoner and given a stigma to indicate that. This is surprising since males aren’t chosen as summoners due to their low amount of mana compared to females. As the first male to be a summoner Kazuki is under a lot of scrutiny by almost everyone.

To complicate matters his sister (who’s a magical swordsman) is against him going into summoner school to train there as a summoner. Because summoners and magical swordsman have a cat mouse thing going on where summoners look down on magical swordsman as inferior so his sister naturally is against him going there. She’ll stop him even if it means going to war.

Kazuki however is determined to thrive in summoner school and has already determined to give it his all. Even if he’s the lowest ranked student (ranked E) and his fellow students intend to demean him he’ll prove himself as the strongest summoner.