Jay's father
You never see Jay's dad until the very end of the movie. Twice in fact. I'm pretty sure the man trying to kill Jay in the pool was her father. At the end of the movie in Jay's room, there's a photo of her family. I'm convinced that the picture of her dad in the photo was also the man throwing stuff in the pool that was eventually shot by Paul. He probably cheated on his wife and then was eventually killed by the thing.
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