Teenage Mercenary (Mercenary Enrollment) is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on “readmercenaryenrollment.org” fastest updating comic site. The Summary is

When he was eight years old, Yu Ijin was the sole survivor of a fiery plane crash in a foreign land. To survive, he became a notorious mercenary—honing his arm-to-arm capabilities, marksmanship, and combat prowess into those of a skilled predator. However, 10 years later, after encountering soldiers deployed from South Korea, he rediscovers his remaining family: an adorable little sister named Dayun and a warm-hearted grandfather. Upon returning to his home country, a brand new trial is presented to Ijin—to assume the mundane life of a typical high school student. From scheming bullies to newfound friendships, every experience is new for Ijin. However, when his past begins to catch up to him, Ijin will have to use his years of experience on the battlefield to protect his loved ones.