itch.io

You can get it when you have five glowshrooms in your inventory (You can find it in the nortcrest cave (south entrance) when you first visit the place or in the lake of purification (when you enter the location there's a chance to find those)). With five go to witch or priestess. In the future updates there will be other options too. 

Yeah, not every sprite is ready. So you won't always see your opponent. 

You can have a guide in the update log (at the end of it), it'll tell you all you need to know for having them (but you'll have to finish the bandit's camp story to have them).

 And for the characters, which ones are missing ?