If you intend it to resemble a NES game, you'll have to acquaint yourself a little more with the NES's restrictions on graphics.

You'll need to use colours from the NES palette, and you're limitted to how many palettes you can make, and how many colours are in those palettes, etc. Your tiles and sprite currently break these restrictions.
Aside from that, character lacks contrast. Pick a strong highlight and a strong shadow and shade according to a lightsource. This will lend contrast and a sense of form to the subject.
Why not try some secondary motion, such as having the tail trail behind the motion of the main body of the figure? Or the raised hand trailing behind in motion. These kinds of things can add a lot to the impression of real physical motion.
What kind of environment is this set in? The ground looks cool, but what is it meant to be?
ack... i though i could set up my ow 56 colors.... well, i guess that i don't have the other chooise to use them... ho well... ^^;
for the sprite, there a certain limit that i can use ? i know that i should use a 8x16 and 8x8 only. so, how can i redo them without losting the detail ?
for about the animation, i will try to improve it a bit more but since it's limited to 4 frames per move, it will be kinda hard ^^;
edit : i forgot to mention it, i wanted to make in kind of outside place near the forest... for moment, the background is awfull lol ^^;
The main thing for a nes game is really that three colors+transparency each 8 pixels.
i knew it, i took extra attention on the number of color, i should use 3 color per 8x8 tiles.
by the way, there the new progress of my little animation :

any critism and comment please.