In gameplay terms, it is the 'MP' or 'Mana' of the game which is needed for each attack. If you have no MC, you cannot attack.
Releasing karma of folks can lower their MC consumption. Ellen's final Cloak of Midnight Sun also reduces the amount of MC needed to attack with.
Both characters' MC recharge is different, and the player may prefer one over the other.
Ellen: Recharges slowly at a constant rate
Keats: Recharges to max instantly when idle for a moment