This is a pretty good proof of concept for a platforming game. The experience is quite short so I don't have too many notes, but here's a few.
- Depending on how the final game will look, I envision the shadow mechanic maybe having some issues. Right now it's a pure enhancement of the moveset, which will force the level design to become very obvious (you'll be forced to put crystals wherever the shadow form is needed, which will probably tell the player exactly what they need to do). This can work if you're envisioning a celeste type platformer, but might run out of legs, especially if the plan is a more metroidvania style game.
- A better approach might then be to instead give both forms their own abilities, and let the player figure out which form is required in a certain location, ditching the crystals entirely. Right now you just want to be in shadow mode whenever you can be.
- If you decide to keep the crystals, you can maybe do some interesting stuff by extending the form when you continue to pick up crystals. That can create some challenges where the player needs to keep up the pace to maintain their form.
Other than that, I like the visuals and the audio and music are a good fit. There's a lot of ways you can take the concept from here but the groundwork is pretty nice. Good job!