I Breathe Euro - Chapter 205: Aether Energy.

When an S-class elite assassin-raised captain of a special ops suicide squad comes home from a high-stakes Africa mission, he finds himself offered something that could potentially set him up for life- a curious assignment that he could use to finally exit his days of war and slaughter.

Then placed into a high-school setting, Lin Yi is tasked by the chairman of a top-tier multi-billion company with bodyguard duty for non-other than his own daughter, a certain little Miss of said high-school.

His enemies, once ranging from drug lords, mafia bosses, political figures, and national leaders, are soon replaced with spoiled kids who happen to have rich papas and mamas, and Lin Yi finds his intense expertise and high-threat skillset misused, and very underwhelmed…

But he does get a harem cast as diverse as his old enemies, so Lin Yi decides that, for the sake of messing around with girls as an invincible entity in a down-leveled environment, leaving his bloody and death-filled war days behind is something he’d definitely want to strive for.

- Description from Novelupdates

A man reincarnates in the body of a boy who suffered a tragedy, only to realize that he is in the world of My Hero Academia. What's even stranger is that his quirk seems to have the abilities of Nen and Haki.

Han Yang never thought he would one day sell himself into a marriage with another man. He believed his life would be mundane but that man, named HeLian Qing, broke his pre-planned life. Parental debt, a child’s compensation, he had no chance whatsoever to resist, he has no ability to resist.

“Hey monster, you're not ready for this blow, but I don't give a shit!!” said Lian as he drew a circle inside another circle in his mind.Then, a few seconds later, a fireball as big as a house hit a monster just over 4 meters tall...It was a waste of resources, but it was damn fun!“BOOM!!!” he said as the fireball reached the monster, covering it entirely.The excitement on his face was palpable as his clothes and hair fluttered from the power generated by the impact.[Accuracy: 97%]“Tsk, it's really hard to achieve 100% accuracy when casting spells... that just means I have to do better!”Meanwhile, the monster and the entire area around him was destroyed. The crater was huge, flames burned the ground, and ash mixed with dust covered the air.---In a world full of Gates, Mages and Knights work together to defend humanity from extinctionLian, a boy from a small town, awakens a very... unique system.Casting spells by drawing strange shapes in his mind had become normal for him by now.Thanks to the system, he had become a Mage, a human capable of using mana to cast spells.Facing monsters from the Gates had become routine. Studying magic was a pastime. After all, he had the System, he didn't need to study classical magic. Or maybe not...