Warning: too many spoilers to hide behind a tag.
Trevor must be a demon.
The Good Place where our heroes reside is a fake Good Place and is actually a Bad Place, run by demons. The Janet is a Good Janet, stolen from an actual Good Place, but all the others are demons (and a Bad Janet).
Since Trevor participates in this, he must be a demon, either from Michael's cast of demons for his Good Place, or one from another neighbourhood in the Bad Place. Note that the animosity between Michael and him might still be genuine; Michael seems to have a predilection for more sophisticated ways of torture (hence the whole Good Place set-up) while Trevor seems to like banality and rudeness.
The Medium Place doesn't seem to be reset when the Good Place is reset, since when Eleanor and Chidi visit it during attempt #802, Mindy St. Claire tells her she has been there 15 times before.
So it is likely that the Medium Place is not a part of Michael's experiment, even though he might be using it.
That means that the orientation video for Mindy St. Claire must be real, too. It shows the interaction between Bea from the Good Place (which might make her an angel), and Trevor from the Bad Place, a demon.