Looks nice, but you've changed the proportions. The legs look thicker and the torso seems a bit wider. It'd be nice if it could be used with the rest of the sprites (which would also mean animating it )
There aren't really that many shortcuts you can take. It's going to take drawing skills, general shading skills, and most importantly, lots and lots of references to ensure you're matching the style.
The image you posted isn't a bad start, but there'a bunch of things you need to adjust. Like Felo said, your first step should be to resize and redraw the character to match SNK proportions more closely. Alpha style proportions are waaay too exaggerated (in part to make attacks more readable). These include but are not limited to: The gigantic hands, the overly puffy cargo pants, and the really thin ankles that end in gigantic feet.
None of those things are elements you'll find in KOF sprites. Additionally, the lighsource in SNK sprites comes from the top right (top left for P2), whereas Capcom tends to light their characters straight from above. That means you have to change the lighting as well.
Last but not least, the references I mentioned. Thanfully, you have A LOT of great examples in this regard. Namely, Ralf, Clark and Marco from NGBC. All of those characters wear outfits similar to Nash's. Also, Guile from SVC can help as well (even though SVC SF sprites can be a little funky, it's the style you'd be pairing Charlie/Nash with).
Thanks ! I struggled with animating with Photoshop, I should try more!
GraphicsGale has a real-time preview that keeps playing over and over and as you edit your sprite, you can see it change in real time in the animation. Not just that, but you can slow and speed up the preview itself to look closely at things without having to change the animation timing in the actual frames themselves.
Not saying it's better than Photoshop or anything, just pointing it out for the sake of awareness (and for people who can't afford to get Photoshop).
There should be a lift for each leg. Right now you have one lift per cycle, which makes it look like she's limping on one leg. Here's a quick and dirty edit (the images you posted are resized, so I can't help you with the original sprites):
This fixes the up and down issue, but the ribbon movement and hair movement are not correctly synced yet in my example. The moment the body goes down is when the ribbon and hair should puff upwards and then as her body rises, both elements fall down as they're being pulled upwards again.
You might have to be more specific than that. For example: The graphical style you're looking for, how many frames of animation are you expecting, the scope of the project, etc. Otherwise, it's like saying "I have a job, but I can't tell you what it is until you say yet, nor how much it pays or how long it'll take".