Oh man, I've been wanting to review this baby since I started reviewing stuff regularly on my page, and before you keep on reading, this review is going to be an overwhelmingly positive one, I highly recommend this manga, and to this day I hold as one of my favourites, not because it was my one of my first reads, but because it was my first time having a philosphy lesson thrown at me while reading a manga.

If I had to pick through flaws in this puppy, I'd say the side characters (not all of them) are just not bad, they aren't something horrid but they don't have enought to stand out so if that bugs ya do your thang.
Other than that, we're smooth sailin'.

Now we'll get to the meat of this manga, you wanted to see blood right?
well there's plenty of gore to satisfy your sick needs, and if you look at this manga's surface, it's a really cool, action-y gore manga with tons of body horror, and that might be enough for you, but personally, I find this manga's strengths lies in it's character development and portrayal and definition of humanity.

Now when I say character development, I'm mostly talking about out protagonist Shinichi (AKA Big Shinch) and his right hand companion parasite Migi,

these two have excellent character development (and great chemistery between them) and although both are considered protagonists, they mirror each other in a way an antagonist usually mirrors our hero (i.e Johan Liebert, Griffith etc.), and in a very subtle way trade traits from each other, while Shinichi slowly loses his humanity, Migi grasps it and becomes more human.
That parallel is the strongest plot point in Kiseijuu in my opinion, it slowly builds and finally when it becomes most obvious, the manga's climax marathon enters.

Now for the hard part, how do you define a human?, a question old as time, when something is presented that perfectly mirrors human behaviour, is it human?
Some would say a human's intelligence is what defines us as a species, but again, the parasites presented here perfectly mirror our intelligence and even go beyond us.

This brings me to the message(s) of Kiseijuu, which hits like a truck after reading through this, in this universe, we shouldn't generalize, even a parasite can feel raw human emotion and even a human can lose his sympathy, plain and simple.
You cannot define all humans under one explanation, and parasites are our perfect mirror.

The definiton of a human is so general that, in the end, it becomes the most special thing on planet earth.

Thank you, Iwaaki.