He was 14 years old. BK Books

Product description ※Please note that product information is not in full comprehensive meaning because of the machine translation.

The brave young man and his seven friends successfully accomplish the goal of subjugating the Demon King, a long-cherished desire of mankind. However, immediately after that, five of his greedy friends betray him, and three of them, including the brave man, are brutally killed. The brave man who faced the death thinks that he will execute the "revenge" of the friends who were killed together, and that he will take the lives of the traitors and protect the honor of the two friends who were ruined. Then the brave man will be born again with his memory retained, and will be reincarnated as an ordinary girl, Zaradin. Zaradin swears that he will be one of his former friends his former friend and his sworn enemy!