Did you save this as a JPG at some point? There's a huge amount of colours in this, it all looks a little blurred. Could you please upload a clean version so that people can inspect your pixel work and make edits?
The character on the left appears especially blurry. If that's not a compression/saving artefact, then try using fewer colours so that each colour gets a nice big area to do its job.
You seem to be shading each part of each character separately, instead of taking into account the overall form of the character, so they look a bit flat. You're also adding highlights almost everywhere, which means every part of the character is clashing for the viewer's attention. Impose a visual hierarchy: use greater contrast (=using both shadows and highlights) in important areas, such as faces and the character's fighting tools (fists, weapons) or whatever their "signature" feature is; use lower contrast (stick to just shadows and midtones, or even just midtones) for anything that's not so important.