Parasyte 1 by Hitoshi Iwaaki

In a world where mysterious alien parasites descend upon Earth, a high school student named Shinichi Izumi becomes an unwitting host to one such creature. Unlike other parasites that take over their host's brain, Shinichi's parasite, named Migi, only manages to inhabit his right hand, leaving both entities with their consciousness intact. As they navigate their unusual coexistence, Shinichi and Migi must confront other parasites that pose a threat to humanity, all while grappling with the moral implications of their existence and the blurred lines between human and alien life.

Purchase from Bookshop.org