EXCELLENT.
A few tips.
I would do away with the "super flying" jump, and add a more realistic approach to getting highground.
Work on your character icons--I'm certain that with a little more time and effort, yours could be the stealth game newgrounds has been waiting for.
If at all possible, add some techniques! Sure, attack from behind and kill the enemy, but let's see the stealth kill play out.
Weapons; More weapons would be a definate plus. Why move to the ground to assault your opponent, when you could always tag him with a shuriken at a distance?
And finally; Enemies. The whole enemy ninja concept was good, but I for one enjoy chopping up the occasional samurai.
This just made my favorites list.