Rick 2010 looks a bit off, as if his body parts are all seen from different angles. Both hands are facing the screen when he idles. I realize you might have reused the hand sprite, but the arm holding the weapon looks anatomically strange, and the pose is unnatural. your arm would start hurting after a while.
I praise your efforts though, just thought I'd share that as a fellow 2D artist. Music is on point, gameplay seems to be ok, although as mentioned before, some enemies need the offsets fixed.