Well, it took me an embarassingly long time to get to this one...
The art is definitely the standout here. The character art is really, really good, but the pixel art environments and world sprites are fantastic, too. The key art and title card are professional quality and really draw you in.
I'm not sure what Umi is, though. I get that Mimi is a bunny person, but what is Umi? She looks kind of like a robot.
Sometimes sounds didn't play. I blame WebGL. I did like the audio design in general, though.
I found the controls a little awkward (played with keyboard). It's not terrible, and I'm not sure how it could be improved, but it did feel a little weird. The dialogue system adds to the awkwardness- it seems to be advanced with Enter.
Other than that, the platformer mechanics were really solid. Power dash is a lot of fun, air control seems about right, climbing works well. I found the movement a tad slow, but that's really nitpicky. Combat isn't anything fancy, but it works well.
I didn't realize I had an inventory; I was wondering why I was picking stuff up but didn't know I could do anything with it. I'm not sure if I missed the tutorial on this or if there wasn't one.
I wish this game had some sort of state save/continue feature. Length-wise it seems to be on the edge of really needing it, though I don't know how far into the game I got.
The difficulty was ultimately just too much for me. I don't think there was anything particularly cruel- maybe too many blind jumps, but that's all I can think of- but I'm not all that good at platformers. The first level really pushed me to my limit and I gave up a few minutes into the second.
I felt the transform mechanic was underutilized; I only used it to traverse very obvious tunnels and occasionally to dodge projectiles- but it might get used more later. It is a really cool mechanic, though- how many magical girl games let you transform at will?
I can nitpick a lot of things but most of them boil down to "XCVG sucks at platformers". A very nice game overall.