Among other things, the major element you are missing is Timing:


Read thru this:
http://minyos.its.rmit.edu.au/aim/a_notes/anim_principles.html

Although directed at film animation it is still applicable to other mediums.
Play around with some of the ideas it gives you.

Atmospheric Perspective will give you more depth by modifying the max and min distance perceived.
It will also help control focus in the composition:


Scientifically atmospheric perspective is caused by "air particles" compounding over distance.
Greater distances cause them to appear increasingly opaque:


Artistically atmospheric perspective is a collection of perspective cues working simultaneously:
1. Change in size
2. Overlap
3. Hue shift
4. Loss of detail
5. Compression of color range

This is typically simplified to:
Greater distance = Color shift to Skybox color

It's up to you which cues you want to include and how exaggerated you want them to be.

In general:
More exaggerated = Greater distance

Upon closer inspection you have a tiny bit:


But it does very little to separate the figure from the environment as can be seen by a greyscale:

I think part of what makes the concept art really strong is the lighting effects from the sky.


It gives the forest a really nice mood.
I interpreted it to be "old, dead, and a little spooky, but also beautiful".

The very high and low end keys seem to be absent from your mockup.

Although we are unable to see the sky from such a top down angle, you could still imply its color and light using overlays:


So this isn't actually a pixel edit.
I used photoshop and vectors.
Please don't beat me with sticks.

You could execute such lighting effects with some rather simple Shaders.
Talk to your programmer and see if your game engine is setup to handle effects.
If you are interested in the math involved I can explain.

What I've added is probably a bit too fuzzy and may not convey the correct mood you are trying to create.
But it illustrates taking a step closer to the original painting in terms of color and key range:




Although now that I look at it both my edit and your mockup are missing the really dark darks.

I added the spirits because of these:

Most likely it is intended to be light filtering through the trees.
They are interesting just because of their shape and contrast.
In my mind they seemed animated so I converted them into spirits.
But feel free to ignore them if its not what you're going for at all.

The art itself is very appealing.
The colors are well picked.
I don't completely understand the character, meaning "what can she do?", but that will become clear when she is animated.

Great work.
Keep going.

My vote goes to the River City/original sprite from this thread, because they actually look like they're about to fight someone.

Although it's good to point out what a fighting stance should actually look like I have to disagree.(but no offense intended jams, very valid point )
The RiverCity pose is very generic, which actually works well in game since the player does other things than fight.

Here we see the player shopping, talking to a child, and engaging an enemy:


A few years later Technos made sprites that have much more attention to volume and still retain the same style and charm:

They also have a good variety of fighting poses that read really well.

Now let's drop one of those sprites into RiverCity.
Here we see the player threatening the store clerk, ready to punch a kid, and about to engage an enemy:


It's good to have a "generic action" pose in your sprite sheet so that it can pass off as just about anything (yes even skiing!).
If it feels too generic for certain actions however, like jams is pointing out, you'll have to add an additional pose that is more specific.
And of course the opposite is also true. If a pose is too specific it could give the wrong impression.
So it's good to have both.

The new post is a good improvement!
His volume, lighting and color are all much more appealing.

The crotch is a bit forward facing compared to the rest of his body.
You may want to consider bumping it over a few pixels:

How many pixels exactly?
It's up to you.

The walk cycle is a good start.
But I'd like to address it in a separate post, as I'm a bit tired.

You're making great progress. Keep it up.