Valun is a demonic-like monster who currently has partial-possession of his human host, Keiran. He is a creature whose possession manifests as severe illness, which can quickly kill the host if the symptoms are left untreated. He also has feelings for his human host, though is very conflicted about them and is tempted to just kill Keiran to be over with it.
The collar is an indicator in his species that he is still tethered to a human host. While Valun can shapeshift and even disguise himself as a human, this collar is always present around his neck until he takes full control over his host (or until the host dies)

Valun's ToyHouse info: https://toyhou.se/22315413.valun
Valun's Artfight Page: https://artfight.net/character/3614328.valun