The brother-sister dynamic between Tanjiro and Nezuko Kamado in Demon Slayer is similar to Yu Ijin and Yu Dayun's relationship in Mercenary Enrollment except the latter takes place in the classroom and Dayun is, unfortunately, not a demon.
In Demon Slayer, Tanjiro Kamado's life is thrown upside-down when he returns home from selling charcoal to find that everyone in his family had been massacred except his sister Nezuko. Despite surviving, she had transformed into a demon, though, luckily, Tanjiro learns she still retains some vestiges of her humanity. Tanjiro, being the good brother that he is, takes it upon himself to make his sister human again and sets off on his long journey, carrying Nezuko for most of the manga in a wooden box to protect her from the sunlight. Although incredibly powerful as a demon, Nezuko can only regain energy by sleeping for prolonged periods of time since she doesn't consume human flesh as is customary of and a more efficient method for demons, forcing Tanjiro to protect her.
This touching relationship between Tanjiro and Nezuko is emulated to some extent in Mercenary Enrollment when the Yu family's long-lost brother and grandson Ijin returns home after it was believed that he had died along with his mother and father in a deadly plane crash 10 years earlier. Ijin ended up being the sole survivor of the crash and spent the last decade as a mercenary to survive, unbeknownst to the rest of his family. Upon Ijin's return home, he quickly learns that his sister Dayun has been subjected to such horrific bullying by certain classmates that her suffering has essentially become ubiquitous at school. This fills Ijin with so much rage that he endeavors to ensure no one bullies her ever again ... by force.
Much like in Demon Slayer, this effort would prove to be a much more difficult task than originally intended, and things escalate quickly. Most of the violence against Dayun may have been perpetrated by a female classmate named Heejin, but once Ijin starts protecting Dayun, Heejin calls upon the help of her older brother who just so happens to be the leader of a feared school gang. When Ijin successfully clobbers her brother and his gang twice, Heejin gets her father involved who's, of course, a powerful congressman. Her parents immediately take advantage of their vast amount of connections by contacting a man known as the director who is feared and revered in the criminal underworld. But, of course, Ijin takes them out easily before actually daring to confront Heejin's father, the congressman. Ijin first defeats the congressman's professional security team and is able to blackmail the congressman with incriminating evidence of his children, forcing Heejin's father to ship her and her brother oversees. And this is only within the first 10 chapters. There are more than 30 that have been unofficially translated into English.
Even though Demon Slayer's Nezuko Kamado is a powerful demon who can hold her own in battle, the fact that she must sleep for prolonged periods of time requires her brother Tanjiro to protect her, using the powers he obtains by first training under Sakonji Urokodaki before joining the Demon Slayer Corps. This dynamic where a brother protects his sister is successfully mirrored in the highly engaging manga Mercenary Enrollment as Ijin goes above and beyond to ensure his sister Dayun is no longer bullied. Also aiding him in this endeavor are some highly useful skills that he acquired when he was a mercenary instead of as a demon slayer like Tanjiro. Regardless, both brothers care for their sister greatly and will go to any lengths to ensure their happiness and survival, even at their own expense. Tanjiro just has to travel much farther to achieve this goal while Ijin can accomplish most of what's required right in the classroom.
