Aesthetically this is very cute, and the mechanic concept is pretty interesting with the ability to join the characters' abilities.
I do wish there was a button to manually split, as there's a number of places where you can merge before you're intended to (especially since most levels start you merged then split you at some seemingly-arbitrary time) and once you get to stars that need certain colors that can make levels unbeatable. And as others have said, restarting all the way from the start for any failure is needlessly frustrating.