Parasyte offers up a nice balance of elements of sci-fi, horror, and psychological drama in its exploration of the dynamics between humans and parasites. The first several episodes of the series start off fairly light in spite of the messy ways that parasites and humans get killed onscreen as the series milks some humor off the awkward interactions between Migi and Shinichi as the former tries understanding humans and the latter tries concealing his rather unique predicament. However in a rather shocking plot twist which I won't spoil here, events escalate toward more darker and serious fare when Shinichi becomes more willing to seriously engage the alien parasite threat and learn more about their presence on Earth.
From that point, Parasyte mixes around focus between exploring more about alien parasites and getting into some existential themes regarding the worth and relevance of humanity as a whole. More is revealed about the motives of parasites like Reiko Tamura who have varying perspectives on how to coexist with humanity and an apparent alliance made between a number of them that gradually become curious or threatened by Shinichi and Migi's unique co-existence. The events also drive Shinichi to come to terms with comprehending humanity's worth in the world as he tries to understand the varying perspectives that parasites like Migi have regarding human concepts like emotions, procreation and desire to bond with other humans. Both elements mostly carry gray morality to them as the worth of a living being's existence is one that is not an easy question to comprehend and Shinichi finds himself coming to struggle with trying to understand what he has become with his coexistence with Migi.
In terms of style, the series also creates some rather convincing suspense out of Migi and Shinichi's fights with random parasites. The two find themselves often having to strategize and create plans on the fly to exploit the environment and openings against enemy parasites to get an upper hand or fatal blow delivered on them. Many of these instances are risky for the pair of them and they usually have to deal with minimizing human casualties during more hostile situations, which raises the stakes and tension during battles. Outside of creating genuine thrill out of whether or not the two will prevail in their fights, it also helps to further develop the relationship between the rather unique pairing.
All isn't perfect with Parasyte though. The romantic relationship that the series attempts to push with Shinichi and Murano is not quite all that convincing as Murano has little involvement with Shinichi's struggles throughout the series and she does not get much in the way of focus throughout the show beyond being concerned about the outward changes with Shinichi's character. The finale also suffers somewhat with a couple villains introduced in the final episodes of the series who are rather shallow with their motives and lack much in the way of dimension, even with the genuine suspense created with the danger they offer up to Shinichi and those close to him.
Overall, Parasyte offers some engaging suspense and psychological drama with the relations and hostilities between parasites and humans seen through Shinichi and Migi's eyes. In spite of some of its flaws, the series is still worth checking out if you're a sci-fi and/or horror anime fan.
