Here is what I have figured out of the mechanics.
You get one point for each orb you pick up, and use up one point to cast a spell.
Mercy counts the number of monsters you chose not to kill
The spells of each character to the best of my knowledge are:
elf: kill monster
red blob: poisonous gas
sprout: switch characters
blue kirby: paralyze monster a turn
butterfly: pushes monster away
page with 2 eyes: ?
purple thing: replace self with potted plant
hydra: swap with monster
mr. skipped leg day: monster reverses direction for a turn