The graphics of the game look great, and seem to fit well in the style of the classic Gameboy.
The sharp diamonds that surround the stage really seem to compliment the blocks that the stages are composed of.
The bricks of the backgrounds look varied with missing bricks which goes a good job preventing the player from noticing repeating textures. Perhaps you could add some cracks in the bricks as another detail in the background as well?
The way that the character's cloak folds as the character moves around is a very nice detail in the walkcycle. Normally most cloaked character's cloaks would remain static as they move around, but this is an exception.
It's great design that you teach the player the mechanics one step at a time instead of just shoving all the instructions into the players face at once. The instructions consist of only one word each time, so they're quick to read and it's expected that the player should figure out the details of the controls themselves.
The controls for the player feel nice to navigate around the levels with. The player has a slight bit of a slide which makes the horizontal movement controls feel smooth. I noticed that you have implemented coyote time into the jumping mechanics when I tried hopping off the corner of a platform; this is a great design choice for a platforming game. One thing that could be adjusted would be to loosen the raycast area of the ground check. It feels like it requires a bit too much precision in order to time my bunnyhopping correctly.
The skull being the goal of each level doesn't seem too fitting with the cheery music and cartoonish graphics. A simple door with a lock would have not only been more fitting in the game's style, but it would make sense as to why you are getting a key. A key and a door seem to match more than a key and a skull.
I seem to be getting slightly motion sick seeing the aggressive shaking every time I beat a stage. The aggressive quakes don't seem to spark the mood of "victory" to me, they seem to convey more of something large and dangerous occurring, such as an earthquake, large traps activating in ancient ruins, or an evil colossus awakening. Having some simple particles flying out should be enough to show the player has beaten the level.
All of the puzzles are way too easy to solve. Every puzzle just consists of moving forward, and then rewinding until the key reaches you. I was really hoping for some sort of puzzle that was different, but no level had any variety with reaching the key, other than to use this one strategy.
I was also hoping that the game would consist of more puzzles would have more variety on how to not only carefully navigate the key, but to also to navigate the player themselves. One game I can think of right off the top of my head that does this right is Donkey Kong (1994). This game is VERY similar to the one you have done for the jam and I think looking at this game's level design would help improve the layout of your stages. The majority of DK 94's levels involve Mario needing to get a huge key to a locked door by solving platformer levels that also incorporate puzzles into the design of the levels. Mario usually needs to navigate through large sections that consist of many platforms before he's able to reach the key. In some other levels however, the game changes it up by making the key is completely out of reach. A few different contraptions need to be activated in order to find a way to reach the key. Some examples of this include the key being on an inactive conveyor belt, and Mario needs to pull a lever to activate the belt to move the key towards him. Another example involves Mario needing to activate some bridges to make it to the key.
Overall, while the game does have great-looking graphics, smooth controls, and a fitting soundtrack, the level design could be much more varied, and challenging for the player. I'm really hoping this review can help you improve your game in the future.