Boo is neat, though I do have some thoughts regarding their capabilities.
Boos can target things through transparent things, like glass. This is kinda neat, making Boo able to sometimes teleport through glass or grates, but if there isn't a target to get OUT, you can softlock yourself. Considering Boo Mario was able to phase through grates at will in Super Mario Galaxy, we could just give that ability to Boo here. May be easier said than done, but would be a neat thing to have.
Boo can't break things or spin under certain obsticles. This COULD be terrible for Boo, but luckily you can still pathfind your way through the game from start to finish. Making Boo smaller would help the ghost duck through more paths, but Boo may not need it.
Boo being able to harm things without teleporting is very much needed. Pretty sure the final boss would be a softlock otherwise, so it's good you can just kinda float into things.
Boo can hang on to the conveyor belts in Arid Canyon, but can't get out unless the poor thing uses all it's rings hiding. It can't really jump out.
I think it would be neat if Boo could just... phase off the minecarts. Would be a funny way to skip all those Minecart sections.
Boo being able to stay grounded is kinda odd to me. I blame Boo Mario, he bounced when he touched the ground.
Boo and anti-gravity is wack, to say the least. Falling and rising don't quite seem to work as intended when the spooky ones go upside-down.
Teleporting into the wall-snails kills you. Maybe don't let Boo teleport if the ghost would die doing so? Would be a nice quality of life thing.
Hiding is underwhelming... for stealth, at least. You stop ALL your momentum, and you just kinda stay there. I tried using this against the final boss and... it didn't seem to help much, to say the least. And, most notably, if you hide in the air, it takes forever to get moving again. Bosses and enemies alike seem to mostly aim for where they last saw Boo before hiding, which ends up just being where you actually are, making it a very... NOT useful stealth thing.
Stealth is overrated, though, and hiding has one amusingly useful feature: a hitbox. This lets you do somewhat odd things, like hurt enemies and blocking laser beams despite Boo being transparent, which I would normally assume to be Boo trying to phase through stuff, not tank it.
Finally, it's perhaps TOO easy getting score as a Boo. If you never touch the ground, your combo never resets, allowing you to gain absurd amounts of score, especially in Greenflower.
Overall... Boo makes for a neat character. There isn't too much to the moveset, but I feel there doesn't need to be.