A New Life of the Teenage Mercenary

Isaac was once a teenager who ended up being an orphan when he was the only survivor of a car accident where his parents were also.
Years passed and young Isaac became a full-fledged boy. However, when he was on his way to visit the grave of his parents, he ended up dying.
But, that was not the end of his life...
Having reincarnated as Yu Ijin from Mercenary Enrollment, his new goal will be to protect his beloved sister and grandfather.
Unfortunately, the path to achieve his goal is full of obstacles and many difficulties will arise, will he be able to overcome them all?
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(Author's thought: Obviously I don't own the rights to the works mentioned in the fan-fic, this is just a project out of my head and thought to be able to write the changes I wanted to make to the stories!)