Does Beetlejuice have character traits, or is he only a demon who acts according to his nature?
He has a very clear intention: say my name three times so I can be loosed onto the world. Whatever he does at the beginning of the movie is in service to that want. When he’s free, though, he pursues mayhem and tries to marry an underage Lydia. Sowing chaos and practicing sexual assault are pretty much demon actions. It’s quite possible he’s a Null.
Does he ever exhibit a characteristic that is distinct to him and not a generic trait any demon could hold? Is Keaton only portraying an imp, a poltergeist?
It’s well-acted and perfectly suited to the story. Beetlejuice is not our protagonist, of course. As an antagonist, he is driven strictly by his demonic impulses. Yes, he’s a Null, and it’s a wonderful choice for him.
However, Beetlejuice 2 is currently shooting. I’ll be very curious to see if Beetlejuice remains a Null. I would argue that his lack of an Enneagram is part of what makes the first movie beloved. I hope the showrunners stay true to that instinct.