Your lightest AA colour is unnecessary, it makes no perceptible difference zoomed out, and just makes it look blurry zoomed-in.
The sprite feels plain/flat for three reasons:
1. There's no shading. Even for a simple style, a little shading can go a long way.
2. It's full of straight lines, which makes everything look stiff. Even very tidy characters are full of curves. Bodies are full of round shapes, and limbs are rarely perfectly straight. Think about the character's personality. How can you express it with their pose? With the shapes used to create the character? With small sprites especially, you also often have to exaggerate. For example, if the character's meant to be hunched over, maybe they should be really hunched over, with their head practically hiding their chest.
3. The colours are dull. You don't have much to play with colour-wise since the character has a lot of black and white, but even just picking more vibrant AA and outline colours can help! When blending between two colours, don't just pick some mathematical midpoint, go through some other hue. Try to avoid using greys (including pure white and pure black), as they're dull. A warm or cool "grey" can convey a lot more without looking too colourful.
Here are some minor tweaks to address these. I didn't change your base colours and mostly avoided touching your lines, instead I mainly focused on adding some minor shadows and tweaking AA/shadow colours. I shifted one of the legs a little for a more natural pose.
(By the way, you don't need "[Feedback]" in your thread title, that's implicit xP Getting feedback is the purpose of posting in the critique section.)