Ethan Matthews (Simon Webster) is the youngest member of the Matthews family and the show's most quietly disturbing mystery. His children's books contained references to Fromville, the lake, and specific locations before the family ever entered the town. a detail the show established in S1 and has never explained. S4E3 introduced a new layer: Jim's ghost (or what wears his likeness) appeared specifically to Ethan and told him to find the basement door. In S4E5 that instruction led Ethan toward danger. Whether Jim's ghost is protective or a trap being laid through the child the town knows Ethan trusts most remains unresolved.
Two competing reads: (A) Ethan is a chosen vessel for some future role. his pre-arrival knowledge of the town suggests the town selected the Matthews family deliberately. (B) Ethan is being manipulated by MIY or the town through Jim's likeness, as a lever against Tabitha and Boyd. The pre-arrival book content is the show's most unexplained canon detail and the community rates it among the top three open mysteries.