Devilman Crybaby

Akira Fudou is the gentlest boy in his school, and that turns out to be the qualification his strange childhood friend Ryou Asuka has been looking for. Ryou tells him that demons are not myths — they are an older race coming back to reclaim the planet — and that the only way to fight them is to host one and override it from the inside. Akira agrees. The fusion gives him the body of Devilman and leaves the rest of his life looking like a slow public crime scene. The series unspools from there, faster and bleaker than its hero realizes, into a finale he can't outrun.