Hi !
Sadly I couldn't find these Treasure-like characters you're talking about so I am not sure what style you're aiming for. I am assuming it's something similar to anime seeing how you made the eye.
I like the contrast slim at the top/wide at the bottom you made. It gives a very interesting shape to the character. The pose is nice and dynamic. I also like the color you choosed for the hair

The main problem on your piece is readability. To be honest, at first I wasn't sure what I was supposed to see. When drawing you need to keep in mind what you want to highlight. It is even more true in pixel art where often you have a very small canvas and no room for details.
Focus on the big shapes by placing cluster of pixels instead of orphans (single pixels). Since you're working on Aseprite get the Preview window (shortcut F7) to always have a zoomed out view of your canvas on the side. This way you make sure everything is readable from the distance. It also helps get a fresh eye on things.
Most of the time for a character you will highlight their face (or at least their eyes) because it gives a lot of information on their personality. You want to make sure the overall shape reads well and if they have an accessory which is important, like in your case with the gun, it needs to stand out.
Here is a quick overpainting I made on your character to show you my process and how I tried to correct it.

1. The eye was about the face size so I made it a bit smaller. I lowered the ear which should be at the eye level. I made the overall body shape and try to get good proportions (it's important even for stylized characters). For the hair I made clusters to define big shapes.
2. For your colors you want to avoid using pure black or white, and only saturated colors. When doing shadow you want to darken the color but also change its hue and saturation or it may look muddy. Color is a wide and complex subject but to make it short I made this cyan-green from the hair goes toward the blue in the shadow to get more "cold" shadows. I did the same thing with the skin color. I also tinted the black to a very dark cyan to avoid using a pure black.
3. I tried to find a color for the green outfit which could go well with the hair. Sometimes it can be hard to find the right color right away and Aseprite has a nice tool which can help you. Select the magic wand (w) and make sure contiguous is unchecked on the top toolbar. Now with this wand when you select a pixel of a certain color you're going to select all the pixels of this same color. Now do ctrl+U or go into Edit>Adjustments>Hue/Saturation and you will have sliders to change the hue/saturation/brightness to help you tweak the color to something better.
If colors are too overwhelming at first it's ok. You can always look for color palettes or for instance just google character with green/cyan hair to see what colors goes well with it. Pinterest is a nice website to find inspirations and references.
4. Only using those cyan colors made this drawing feels a bit monochromatic. Monochromatic isn't wrong but it can be hard to handle and well honestly a bit boring. So I decided to bring some yellow in the outfit. I also got rid of the stripes on the pants which are distracting and taking the focus away from the face.
5. Since you're going for something anime-like I added "black" outlines which are actually the same very dark cyan I used for the pants. It's a cheap way to add contrast and make your character pop on a background. By cheap I mean it's part of the art style and it should not be used to cover a problem of constrast.
It's quite a long post I did there but hopefully it helps ! Keep up the good work
