O' boi, I wasn't expect this kind of review! Thanks! I'll try to address point by point.
Yes, I've failed to communicate that you could buy multiple heroes. This could have been easily fixed with a 0/4 text that increased with each ninja bought. That was a clear case of "because I coded this I know it but I forgot to communicate other players".
Sadly, I did want to improve in character powers but didn't have time to do so. Bullet time ended up being useful only on the 2nd and 4th level, maybe a bit in the 3rd, but it was very situational. I have many ideas of improvements to do to these powers, but they have to wait until the ratings.
I originally wanted to have a dedicated tutorial level, but didn't have time to implement so I opted for the in game texts. I wanted the text to come out of the player because that would made more sense with the ending of the game, but I never thought about slowing time during the tutorial, which now that you say it it's a great idea that I'll definitively implement.
The letters at the top sadly were just placeholders that stayed in the long run because they weren't high priority.
The button placement never gave me any problem. That being said, I wanted to have remappeable controls as well as gamepad support. I'll be sure to add that.
And with the subpixel drawings like sprite stretching and full rotation, for the final version of the game I'll leave them, and I'll only stick with the 4 color scheme (probably adding alpha blending), but it's true that, for the GB Jam, these effects did detract from the retro feeling. The problem is as follows: GMS 2 camera system makes it very difficult to disable subpixel drawings, so I opted to have selectable scale. If you go to settings you can set scale 1, which will give you a window of 160x144. Of course, this makes it very hard to actually see what's going on, but then the rotations and the sprite scaling will be on grid with the resolution, thus making it more gameboy-ish.
Thanks a lot for the review! It gave me great points and I'll be sure to apply them in a future version of the game, as well as (I hope) my next jam!