what a fun idea! i thought phasing into walls was a bug at first until i notice my hp going to almost zero after spamming it to go the ends of the earth xd. the hp reduction is a really neat trade off to being able to pass through walls and skip different sections of the game (also helps inform the player to think about their positioning when phasing). i can imagine how crazy routing would be for speedrunning a game like this if they have to take this phasing sequence breaking mechanic into account!
i also like how fun and playful the sequence in the beginning was. the art in the game is also really nicely colored and drawn (that background especially). great work!