Seirei Gensouki: Spirit Chronicles

Meet Rio: a callous orphaned boy living in the slums. At only 7-years-old, he realizes he’s actually the reincarnation of Haruto Amakawa, a Japanese university student with a tragic past. While still reeling from this shocking epiphany, Rio also comes to learn that he possesses extremely potent magical abilities and uses his new powers to solve the kidnapping case of a little girl. His good deed is acknowledged, and he’s rewarded by being enrolled into… a prestigious academy for noble children...? 

(Source: J-Novel Club)

Amakawa Haruto is a young man who died before reuniting with his childhood friend who disappeared five years ago. Rio is a boy living in the slums who wants revenge for his mother who was murdered in front of him when he was five years old. 

Earth and another world. Two people with completely different backgrounds and values. For some reason, the memories and personality of Haruto who should've died is resurrected in Rio's body. As the two are confused over their memories and personalities fusing together, Rio (Haruto) decide to live in this new world. 

Along with Haruto's memories, Rio awakens an unknown "special power," and it seems that if he uses it well, he can live a better life. But before that, Rio encountered a kidnapping that turns out to be two princesses of the Bertram Kingdom that he lives in. 

After saving the princesses, Rio is given a scholarship to the Royal Academy, a school for the rich and powerful. Being a poor orphan in a school of nobles turns out to be an extremely detestable place to be.