Hosts are a theme in Demon: The Fallen. They are human bodies who serve as vessels for the Fallen. As immaterial beings of pure spirit, the Fallen require a human vessel both to exist in corporeal form and also to anchor them to the Earth and prevent them from falling back into the Abyss.
Fallen choose vessels from humans with weakened or dilapidated souls (e.g. comatose patients, severe drug addicts and suicidal people). The Fallen enters the body, severs the soul's weakened possession of its body and takes its place in the body. The mortal soul is either thrown out into the Abyss or is buried deep inside their own body, leaving the Fallen dominant.
The demons take on certain fundamental psychological traits from their hosts; their memories, their emotions, their view of the world and their passions. Their vessel's identity becomes part of their identity. If a host loved someone, the demon will also love that same person and will often make thralls of them. If a host hated someone, the demon will hate that same person and will use its powers to torment him or her.
In addition to the psychological transformations demons undergo after possessing a host, demons also experience a kind of reawakening. If they find a particular passion in their vessel, whether it's love or hate or envy or any other strong emotion, it provides them with protection from the memories of their isolation and misery in Hell, allowing them to stave of their Torment and preserve their sanity. These passions remind the demon that he or she was once a holy Elohim in Heaven with God.