Absolutely gorgeous presentation - I did not expect this level of polish from a one-man effort. The detailed and aesthetically pleasing pixel art style makes the game stand out amongst others in terms of production value, it almost feels like a well-done tech demo for a full game!
The gameplay is interesting concept-wise, but the actual execution leaves quite a bit to be desired. The main issue is that the core mechanic: switching between dekai and chibi seems almost irrelevant, as there aren't any notable speed differences, you don't need the extra jump height much, and dekai's power is always significantly stronger. Having different types of enemies or maps which necessitate this kind of switching (a deep tunnel that only chibi can navigate, or a tall cliff that only dekai can jump up to) would be a great improvement.
As another player also mentioned, the large timer increase you get from the souls make time management completely trivial as well. The time increase being 1-2 seconds should be enough, in my view.
Overall, I think this is a high-quality jam game. Good job!