Overheal is a perk in All-Star Versus 6: The Nightmare's Reawakening. It functions just like the Team Fortress 2 Overheal effect. It allows you to go over your standard maximum HP (ex. Mario has 125 HP standard but can go to 300 Overheal maximum).
Good effects: goes 275 HP over your standard maximum HP by picking up Health Kits or healing with standard maximum.
Negative effects: Overheal slowly decreases by 1 per second, if you go below your standard maximum HP your healing will decrease by 50% (ex. Full Health potion will heal 50% instead of 100% HP), and Overheal will decrease faster than standard HP.