Just completed the demo, and from what you have it's a pretty solid game. I can see why some people do prefer Sai/Amp over Mitch, however I do see Mitch's potential, especially after the first upgrade. The over world and 'puzzle' room, or SaiAmp Mind room I suppose, is definitely interesting. I can see a reference to your previous game in the check points, the infinity girl (forgot her name) which is cool, wonder if we would run into other familiar characters? it be cool. The facial expressions are pretty well done, and what I imagine is the main antagonist looks pretty scary when she's angry... and ugly, those wrinkles.
Okay so game play. General combat from what I got didn't feel as impactful as the previous game, then again it's psychic powers and bullets, so fine ill accept that. They both have their own strengths and weaknesses which make both of them useful in certain scenarios. While mitch can't make all enemies stagger, I did notice he can stagger, or 'pause' flying enemies which makes him much better for flyers vs Sai/Amp. However her dodge is so much better then his wall simply because of mobility, however I'm sure it will have it's moments. I didn't really experiment too much with skills, but I did see how to use the drones and charge shots to deal disgusting damage as you stated. Platforming is pretty solid, Sai-Amp is the best thanks to her floating and dashing, maybe give Mitch a double jump? Kind of even the mobility out a little?
All in all, this is definitely a worthy successor to your first game so far. When it comes out on steam, ill try and grab it to get a bigger chunk of this game. The art style is still it's unique self, it's funny, characters are cute and the combat is fun. Maybe the story gets to be as dramatic? Can't wait to find out!