This was a nice pleasant game to play! I thought the art-style was really charming! The tween style animation was actually quite nice given the art-style. One thing I would like to note though is that I think it would have been better if your wall and floor tiles/sprites were also done the same way as the rest of your art. As it stands, they look out of place, like they came from a different game (floor/walls have color shading, texture, and detail to them while the rest of the art assets are simple colored sprites with a prominent black outline). Having consistency when making a game is always a good thing! Also just want to compliment some of the small details of the game. Nice camera shake when the player jumps, but I really like how the grass responds when you walk over them. It really adds to the experience and makes the world much more expressive! Nice work, big plus for that!
As for the game itself, I've died more times than I thought would and there were some instances where it felt a little unfair. The biggest offender was after you do the super jump off of the boat, I died to landing on a bat that I couldn't react to twice. From what I read, I understand that you like difficult games to play, but there is a big difference between punishing a player's mistakes (good thing) and punishing a player's lack of knowledge (not a good thing). Player's dying due to their mistakes may be frustrating, but dying to something you had no control over is demotivating and could cause players to not to finish your game. That was the only major problem I had, and the rest of what I'm about to say is just minor and something to consider. Maybe think about making the size of the hit boxes smaller a tiny bit? Since players only have one life, its doesn't feel good or fair when you just barely collide with one of the enemies, especially since you have to go through the whole thing again (but its not that big of a deal since the game was relatively short). It doesn't matter if the hitboxes match the size of the characters, I think it is more important to focus on how players might feel when playing the game, even if it is realistic. If you want to keep the hitbox size, maybe go with the classic three health system. Although it would make the game easier, it would also give added tension to players who are on their last hit point and don't want their progress to be reset, which could be nice. I think either the making the hitbox size a tiny bit smaller or health change would be nice, but don't do both (it would probably just decrease the difficulty by a lot, and it would not be that engaging). Or don't do any of them, its just a suggestion to a small problem that one player (me) had with the game.
Back to the positives, I really liked the floating text and the story. The dialogue with the reaper who is with you on the boat gave me a good laugh and the I had a smile on my face every time I saw floating text explaining what to do or why this part works the way it does. It lets players get to know your personality a bit more and reminds people that this wasn't just a game that was made by big company, but just someone who thought that it would be fun to make a video game using Unity and prove that not all games made in this engine aren't crappy ;)
Overall, this was a short sweet experience that could use some consistency in the art and be a bit less punishing. I got to Cerberus and got to heaven after 15 minutes of playing and I had a lot of fun with it, which is what really matters! As someone who really wants to be a game developer, I really like your attitude on game development and how you are handling/taking criticisms for your game. I can't wait to see where you go from here! Great job!