Well, it's definitely not complete. It could use a story or menu screen, I noticed I can walk on top of doors, walk up and down cliffs, and on the second map screen just walk right out of the game. Other than that I like the feel of this game and hope that while this is your first attempt it's not your last.
Custom character portraits, eh? Well I guess there's absolutely nothing stopping me from making an army of badgers. By the might of Forestdig, Burrowsong, Honeyclaw, and Badgerface! Go go badger rangers!
Something I have to note is that the character, the enemies, the bullets, and the background are all pitch black with bold white outlines. Should there ever be a Reactance 2 I would personally suggest some color coding, otherwise it puts unneeded stress on the eyes and makes it extremely hard to focus for those with poor eyesight. Easy target recognition is key in the bullethell genre, one dedicated to clusters. (Probably among the best flash bullethell games I've ever seen otherwise.)
I rather enjoy this game though I feel a bit guilty because of it. It made me remember I had more important chores I was doing, but put them off and played a game where I had a list of chores and decided to put them off. You have my sincerest thanks for not adding a computer into the game from which I could load up this game because I would do it in a heartbeat and never finish my work.