Great entry!! I really enjoyed the vibe given by the graphics and music, it definitely kept the intense energy up. Transforming into different characters was a really cool idea, too, and I think it could be pretty cool to see this continued into a fully-fledged project ^^ Like others have said, the square seems like the best option at the moment and I couldn't find an advantage for the circle, necessary, but having those options were nice :)
Edited to add: Some invincibility frames would also help out the gameplay a bunch, I feel. I got trapped in the pathway of an asteroid a few times, so I instantly lost all my hearts as it rammed into my character over and over. So, removing the player's collider with the asteroids for a second or two could make it feel a bit more fluid and forgiving :)