It’s not a shonen, despite the title. I’d argue it’s for more mature audiences. The story is a tragedy about a man who is forced into immortality, and spends his days fighting demons to prepare himself to kill the demon who did this to him. It’s definitely got some action, but it’s not really about the spectacle of fighting demons. Most of the demons are caused by bad emotions like grief, anger, jealously, etc. The story focuses on how these emotions demons affect people and the world around them, and the main character learning more about himself and the world during his journey.

I’m not good at explaining this, just watch it lol.