Seems like all the other comments mentioned what I would say.
A small bug, if you hug a wall, you can shoot through it as the bullet is spawned far from the character.
For the difficult, it would be interesting to have a little less health. And for recovering health, return only half way to 100% (if near to 0%, get 50%, if at around 80, go to 90%) I think it would give that feeling of just delaying the certain defeat (better when used with high-scores)