TL;DR: It's a decent little game, but everything just kinda plays too slowly.
* short levels that generally don't overstay their welcome.
* collectable items aren't irritatingly difficult to find.
* fully rebindable controls. (They start off as WASD + something on the right side of the letters, but I rebound them to arrow keys + ZXCASD very easily.)
* level design on boss stages: bosses themselves are easily accessible, so you can practice on them, without going through the rest of the stage. Basically, you can separately practice getting the kimonos and beating the bosses. This is good stage design.
* Basically everything in the game moves too slowly:
* * Left/right movement momentum effects, making it so that you can't turn or stop quickly.
* * Akane grabs the wall if she runs into it while moving downward, and doesn't ungrab it even when you let go of the direction facing the wall, so you can either slide down slowly or slide down somewhat more quickly (if you press down), but you can't just drop down. (Strangely, boosting off of a wall causes Akane to move much more quickly.)
* * Level complete screen animation and music just takes too long to play out.
* * Game over sequence also takes too long to play out, considering how easy it is to fail a stage.
...So in general, it doesn't feel like you're playing a ninja. You don't get to enjoy the agility of a ninja...though it's not like the enemy ninjas do either.
* Edges of platforms should be drawn so that they don't stick out beyond their hitbox (or don't stick out that much). This applies mainly to the forest area.
* Bug: If you are in a corner and there is water flowing against you, you can't simply move left/right out of it. You can only jump out of it. (Please post a comment if this is fixed; then I will remove this item.)
* Soundtrack doesn't loop properly -- it fades out then restarts. (You might notice this if you take too long to figure out the second boss's pattern.)