Welcome to Pixelation.
When I look at your ninja, here is what I see:
I like the shape of the face and hair. Simple, but a good head shape.
I like the shoulder tilt. This makes posing tricky, but it looks more dynamic.
Colors.
A white (or black) background can throw off your sense of values. A lot of folks like using a mid-tone instead. Some use gray 128, some use a slightly greenish hue.
You are using pure black and 3-4 colors very near black. These can probably be consolidated to a single near black color, and then you can use this to connect your color ramps.
Pixels
The lines could be cleaner throughout. chesu covered this in the blog post.
Pose
The character is standing in a neck-breaker pose. Center line of the body and knees face one way, the head is cranked all the way around in the other direction. The shoulder tilt helps, but ouch. This sort of thing does show up in fighter sprites, but usually not to this degree. I think the common thing to do is extend the forward leg (our right) and point it forward. This will also mean adjusting the hips.
Adjusting the pose may mean losing the tilted shoulders, but I think it is more important for the character to read clearly.
Costume
The boobs are too large, even for a busty character. The feet could be a little larger, but not too much.
You may want to change the costume at the hands so that they show up when in front of other parts of the costume.
Hope this helps,
Tourist