Shibito are the outcome of the ceremonies preformed in Hanuda, Japan. They're also present on Yamijima, along with the more dangerous Yamibito variants. Shibito literally means "corpse person".
Because Shibito are, in essence, religious fanatics for a supernatural deity (this deity is different in each game), they feel compelled to fulfil its wishes. This generally includes: Killing anyone who is not yet a Shibito and assisting the deity with whatever it requires to cross into this world.
The red water, present in all games, surrounds the village/island once the siren sounds. Anyone who imbibes the red water will eventually turn into a Shibito - this will happen quicker the more they drink and instantly if they are killed. However, the survivors are able to "sightjack" the Shibito, allowing them to see what the Shibito see, with the same hive mind the Shibito themselves use to communicate. This gives them a slight advantage in avoiding the Shibito, though does nothing to prevent their eventual fate in joining them.
It is hinted in the first game (and its remake) that the Shibito sees the world very differently from the survivors. It is a considerably happier place, full of tiny angelic beings.
Unlike zombies, Shibito are relatively intelligent and retain much of their humanity. They are able to talk, though generally just the same few phrases and folk prayers over and over. They often continue to do what they last did before they where turned into Shibito, endlessly, until interrupted by a living soul. Once they detect a non-Shibito, they will use what cunning and intelligence they have to stalk their prey until they lose track of them, at which point they will return to their usual routine. Many Shibito are armed with the tools (or firearms) they died holding, so should generally be avoided if one is unarmed. Shibito can be knocked down with enough force and incapacitated for a while, but they will always rise back up eventually.
After some time has passed, the Shibito will eventually mutate, becoming more insect-like - in the first game and its remake, this is because their deity is a giant insect creature. This leads to the Fly Shibito (a flying variant that is hard to escape from in the open), the Dog Shibito (a fast-moving enemy), the Spider Shibito (able to climb up walls and across ceilings, often surprising its victims) and the Brain Shibito (who is instrumental in controlling the nearby mutated Shibito and often a key target). The mutated Shibito rise up much faster after being defeated than their more human kin.
The Yamibito (lit: "Shadow People") are considerably tougher (and smarter) variants of Shibito that only appear in Siren 2. Unlike Shibito, they are afraid of light and will not approach a well-lit area. They can also be dissuaded to come any closer with a lit torch, as a strong light will hurt them. Most of the major antagonists of Siren 2 are people that have been turned into very strong Yamibito variants.