This got me thinking what if he's actually hallucinating her and that would be a plot twist
that would mean either he killed his friend or the friend drowned by himself and that he killed the gangsters himself and disposed of their bodies. would be a crazy plot twist...
But couldn't his dead friend see the mermaid? And his friends from his previous school and his sister could hear her too
The friends even heard her talking on that hidden recording so i doubt it
This got me thinking what if he's actually hallucinating her and that would be a plot twist