That was a very compelling experience! I'm usually not a fan of pixel-art, but man, this game looks so good, visuals are top notch! I very much like the concept of playing with 4 slightly different characters on slightly different levels with different challenges, that was refreshing :)
As mentioned in the previous comments, the main issue for me were controls - and I believe there is no way of opening "Controls" menu after you hit "Start game"? I opened the game in the second tab to check it out :D Random thought: I feel like controls for characters can be a little bit more consistent, for example, every character moves right/left/jump and have special ability under E/Spacebar/LMB/Whatever. This way it would be easier for the player to know what ability he can now use, just by pressing action key.
Some smaller stuff:
- Spikes would not enough distinct from the environment for me, I ran on it several times just because I didn't notice it. Also, death animation on spikes could be more responsive/faster, to give instant feedback.
- Adding sound effects would add another layer of awesomness to the game :)
- I'm not a big fan of shooting bubble animation duration, the guy just stays with his arm extended for too long after shooting the bubble, I want to go and jump on that jelly thing, but I have to wait for animation to finish :)
But this is only me whining :D Animations could use some tweaks in terms of timing, but they look very nice and doing good job of communicating the character actions. After I grasped the controls the game was really fun to play! Having different music tracks for every character was a really nice touch, it made it easier to orient myself who I am controlling know and added a lot of personality to each character. And again, the game just looks gorgeous :) Really great job guys!
Edit: I forgot to mention level design, but that's a quick one, because it was really solid, good usage of different abilities, so nice work in here also :D