Teenage Mercenary (Mercenary Enrollment)

Teenage Mercenary manga (입학용병, Ib-hak-yongbyeong, literally translated as Mercenary Enrollment) is actually a Korean manhwa that follows the story of Yu Ijin. When he was eight, he survived a big plane crash in another country. Therefore, to stay alive, he had to become a tough mercenary, learning how to fight and use weapons well. But ten years later, when he meets soldiers from South Korea, he finds out he still has family there: a cute little sister named Dayun and a kind grandfather. So, when he comes back to his home country, he faces a new challenge: going to high school like a regular student. Dealing with mean bullies and making friends is all new to him. But when his old life catches up with him, he has to use his fighting skills to protect his family.