I went through your gallery, though I mostly know your work from your Republic Commando art. Trying to word this, but...Personality? Intensity? I fiddled around with the phrasing of "always seems like something's about to happen" and "coiled spring" but I know that doesn't really work when applied to things like art of someone who's just lazing around and chilling, though I still mean it.

I think intense is the best description I've got, with that intensity always being a constant on a sliding scale between fierce and playful.

EDIT: Wait, you wanted a theme. Um...I guess I'd describe it to someone like: People existing a whole hell of a lot, for better or for worse, and isn't that something.