Please don’t read about Michael if you want to avoid spoilers.
In order to judge Michael’s Enneagram, we need more than Season One. He only begins to change in later episodes. Eleanor thwarts him, which opens up personality options to him. Of course, an argument can be made that Michael should have no Enneagram at all because of what he is. Throughout Season One he tricks us into thinking he has a character. It’s all play-acting, though, as part of his Good Place fake. The real Michael doesn’t appear until the last episode of that season.
During Season Two we begin to see genuine traits. He has an identity crisis. At first he’s only concerned with saving his job, but later he gains an emotional connection to the gang. His victims become his friends. He learns about morality. He tries to do the right thing.
So if Michael is to have an Enneagram, it must be a number that encompasses his beginning as an active demon. For some reason my mind goes immediately to a Five. Fives can have a cold disconnection from others that can seem cruel if you expect a more emotional person. Until he’s taught ethics by Chidi, Michael may have been a competent demon with no qualms. The way Michael is won over by logical arguments also suggests a Head Type.
However, I’m more persuaded that Michael is a Four. He has an emotional breakdown when he no longer knows his role in the fake Good Place. And part of him genuinely wants to connect with the gang. His goofy ability to make mistakes is not completely a ruse. It’s an exaggeration of his true personality. A Four could take pleasure in the dark aspects of hell, and could just as easily switch to longing for a community of friends.
Also — to be frank — a Four is a better liar than a Five. No judgment. It can be a useful trait.