It is definitely possible to be multiple and have more than one person sharing your body without having MPD or DID. Your experiences don't sound that much different from our Jay's when he was first realizing that these presences he'd always felt around him were not "just imaginary friends because I'm stupid and never grew up", but were persons, people in our own right, us.

Being multiple doesn't mean you have to have your whole life fall apart. And your "dark" friend might just be a bit mysterious.

And no, you don't sound crazy at all.