The Smile Entity spends most of the film shapeshifting into the people the victim knows, ultimately manipulating the victim into suicide. It operates via a chain curse: a person becomes the next link in the chain by witnessing the last’s suicide. Though it could be debated whether or not it’s suicide if the person is possessed by or under control of the Entity.
You don’t see the Entity until the very end. Its shape mirrors a gigantic human more than anything – a skinless human, likely taking its cue from Frank in Hellraiser or maybe the creature from The Ritual (in the way it hunches over). Grotesque. Shiny muscles stretched across a skeletal frame.