I'm usually one to zone out and simply enjoy my anime, so I usually don't try to write reviews as I don't go in depth too much, but I learned from this anime so I though I'd try. To begin, the story was fantastic. It's an interesting concept, and the story is perfectly done to intrigue you to ask questions and view the world differently. The characters were the best part. They were dynamic and never boring, with the main character Shinichi and his little parasitic helper forming a bond over the course of the show. The art was perfect for this show but if you don't like a LOT of gore don't watch this. I've seen much worse as far as gore goes, but there is still a reasonable amount in this show so just something to look out for. The soundtrack was suitable for the show, and the opening and endings were fantastic and perfectly captured the mood. The anime was intense and captivating and kept me on the edge of my seat for the first time in awhile, as I haven't enjoyed an anime this much for so long. The real part that made me enjoy the anime was the view it showed me on human beings. We as a race have this sense of entitlement and have these strange emotions such as loyalty, love, and we do things no other organisms do, like suicide. It's like our primary goal isn't just to live as our instinct tells us, but we want to live for others as well. When a predator comes on to earth that threatens human's in this anime, the first reaction is to call them enemies, and to do whatever we can to get rid of them. But what the characters and dialogue makes you think about is things like "What makes us so superior?'", "Why to we get to pray on other animals as a matter of course, but when we get preyed on, the other organisms are the truly terrible ones?". It makes you think of what's truly right and wrong and takes you out of society's norms.
Overall I give this a 9/10 and I'll hope you'll give it a shot.
