At some point of the story, a parasite infects and start living inside this character's body, a process called parasitism.

A parasite is an organism, that lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. Usually the parasite is smaller than the host because it needs to infect it undetected, but can grow larger and even take control of the host's physiology and mental faculties during the course of the VN.

Parasitism only covers physical infections perpetrated by a living organism. In case of metaphysical possessions just use Possession instead.