I didn't get anywhere close to finishing this, but I figured I'd post what I've done so far in the hope of getting some feedback regarding how it plays and any bugs, because I'd like to develop it further in the future.
The title is also the name of the song submitted by valzi which I used on the start screen - it seemed to fit with yokomeshi's transforming cowgirl.
Arrow keys to move, Z to jump, X to fire. Press down and jump to transform - when you're in demonic form you can't shoot, but you can double jump and pressing fire does a kind of dash move. There isn't anything to shoot yet and the level is just some random blocks for initial testing purposes.
Download: possessed_v0.04a.zip
Credits:yokomeshi : player sprites
György Straub : tile graphics
oddball : cursors
Emilio : font
oryx : bullets, green tick graphic
valzi : title theme "Cutely Possessed"
pgil : in-game bgm "Stop Me"
BigLon : shot sound effect
