Despite popular belief, Mari doesn't haunt Basil, the "ghost" in basil's house is actually the immense guilt he feels.

Meanwhile, Mari isn't "haunting" sunny in the traditional sense, more of watching over him, even in death. It's just that what happened the day of the incident has corrupted his view of her, thinking she's wanting something.

Ghosts only appear as what you think they are.

There are a few exceptions, but we're not going into specific ghost types.