I take it that in actual combat, people won't fare too well by doing a full-transformation as it prevents the extraction of crystal hearts? What happens if someone uses a devouring attack on a Yami? Is it still just spilled Essence, or do they get something else?
It would count as spilling the essence, not getting it.
I feel it may have escalated a bit too quickly
Keeping the original purpose in mind this might be exactly as intended. In a long-term campaign there's no way to recover essence lost except to fight the Yami; it would be very easy for a girl to become almost useless because she was badly hurt during a fight and didn't get a chance to recover. Spiking a girl's magical output when she gets wounded would provide a buffer of sorts that gives the player a chance to recover from an otherwise inevitable downward spiral of getting hurt and not being strong enough to recover.
Of course, the question then becomes "do the Yami get to do this too?" Even if the answer is yes then I can see combat becoming very interesting; do I use a death-of-a-thousand-cuts strategy so as to minimize the benefits I'm giving my enemy in terms of available essence, or do I go for the largest and most powerful attacks I can in order to kill them before they can turn their lost essence into more offensive power?
Thats basically the idea here; Since it is gonna be easy to be injured and die, I want these characters to have the ability to retaliate. The downside is, of course, that if you just use up all your essence in a fight, it's gonna be a pyrrhic victory, since you've won that battle but you're more then likely so weak from the effort that you'll die soon anyways.
Alright, lets get some new people in here for this. Lets say...3 people for now. I think I'd like toaster in here, if he'd like to play, since he's my good ol testing buddy. But otherwise, first other two to post a sheet (Stats, magical qualities, natures and weapon) get it.