I've honestly always assumed Esprade's hearts were meant to resemble a life bar... due to how the characters recover from being hit. But who cares, ultimately its a number, when its below 0, game over!
Unless in the case of Deathsmiles it actually IS a life bar that is just segmented into 3 slots, maybe some kind of special attack power meter works off it.. speculation speculation speculation...
Hope we can see some video of Casper and Windia's Spooky Halloween Adventure soon, maybe the fabled Great Pumpkin from Charlie Brown makes an appearance as a boss.
*edit*
And I'm also wondering if the sidekick owl/bat maybe works in some way similar to Guwange... in one of the magazine shots the owl is shooting along with one of the main characters..