In secret, alien parasites drift downwards toward Earth. Their directive: to take control of a human body and thrive in secret. When a parasite attempts to take over Shinji, an ordinary high school student, he stops it in his arm to save his mind. With the strange power of amorphous muscle, the curious parasite strikes an uneasy truce with Shinji: it will keep him alive and strong so that it may continue living, and will help protect him from the other parasites that might not take kindly to Shinji's mind still actively working. Can Shinji gain the courage to face the parasites and protect humanity? And would it even make a difference if he did?
Shiina is a sixth-grade tomboy whose brave and adventurous nature almost causes her to drown in the ocean. However, from the experience she meets a ‘Shadow Dragon' that she names Hoshimaru; strangely silent and star-shaped, it has the ability to shape shift and fly. Soon, Shiina makes yet another new friend: a troubled girl named Akira who happens to also acquire a Shadow Dragon. Yet Shiina and Akira aren't alone - others besides them have Shadow Dragons, and not all of them have such innocent intentions. Furthermore, the Japanese Military has taken an interest in the creatures' combative uses. What are the Shadow Dragons, where did they come from and what is their ulterior motive? Shiina is determined to find out...
Parasyte and Narutaru delve deep into the issues of humans' effects on eachother, though each with very different strategies. Both carry a dark tone but a disturbingly cute sidekick for the main character. And both have twists and turns the whole way through. Good reads that will make you think and squick you out, sometimes too much.
Same author, same drawing style. Both mangas about people possessing alien powers and body deformations. In both mangas protagonists ask themselves whether to use or not to use their powers and other problematic questions. Both are great psychological sci-fi.
Humans who change into grotesque monsters? Checked.
Protagonist who has a monstrous arm that goes out of control? Checked.
Horror with lots of gore? Double checked.
As you can see those two manga share quite a number of similarities, so there is a good chance you might like one if you liked the other.
P.S. I got the impression that Variante is even more disturbing than Parasyte.