My theory of who les revenants are -
OK, it just came to me, watching the second episode of season 2. OK. Bear with me here:
The returned aren't "dead", they're aliens.
The are able to access the memories of the living to give themselves form and exploit past relationships in order to gain access to the people of Earth, which they wish to colonize and with whom they want to interbreed.
They get the emotional thing kinda wrong at times, I think they are also overwhelmed by how powerfully human beings respond to them.
Can't think of any other explanation!
"A sword is useless in the hands of a coward" - Nichiren Daishonin