Alright we need to expand on this guy a little bit. We have a basis but his techniques are a bit wonky. Also I think Boos actually do have genders, with Bowletta being a prime example of a female boo. Maybe not physically, but I think whatever gender they were in their past life they would inhabit mentally when they become a boo.

For adding more techniques, consider what Boos are good at. Sneaking behind people and scaring them, causing mischief and pranks. You showed me a piece of art (feel free to post it in the CS somewhere if you'd like). What about that hauntingly creepy tongue most boos tend to have? We could take a page out of pokemon for example, and say that if he can sneak up behind enemies and licks them, it will temporarily paralyze them with fear. He could also just generally use his tongue as a whip, or some other offensive ability you can think up for your character. 

And another thing; it's true that Boos don't really die, but they can be temporarily destroyed. Making him immune to all but starmen is a little bit much. Let's say he is vulnerable to physical attacks while exposed to light and while not focusing on being incorporeal, but in the dark he is tougher? Resistant to damage, but he has a clear exploitative weakness that doesn't make him too weak. 

These are just a couple of suggestions I have, feel free to make your own changes. We just need to expand on what your character is capable of a little bit.