Some thoughts:
- Cat feels heavy, likely due to her slow acceleration.
- Holding down to jump feels off, handling jump height with just the jump button would be better.
- Enemy fireballs are totally non-threatening, even for World 1. Maybe make them bigger and/or slightly faster.
- Split character system is good and I want to see what you do with it.
- Cool sun.
- Boss fight doesn't feel like much of anything. I'd try integrating the split character system into it, maybe make the fight revolve around solving a puzzle using both characters to beat the boss.