Hikaru ga Shinda Natsu

Plot
Yoshiki and Hikaru are two teenage boys living in a small town in rural Japan. Despite having opposite personalities and different hobbies, the two maintain a close friendship. However, on one winter day, Hikaru is fatally injured while hiking in the mountains alone. Before dying, a mysterious eldritch being comes across him and consumes him, becoming him physically in the process. This ＂Hikaru＂ has all the feelings and memories of the original, yet remains a separate being, something Yoshiki realizes rather quickly. Yoshiki still wants to stay with ＂Hikaru＂; however, his alien nature, along with other eldritch beings and hunters of said beings, may make that impossible.

(source: Wikipedia.org)