The Rage Meter is a battle mechanic first appearing in Riviera: The Promised Land. It is used by the enemies that are faced and it influences the kinds of moves that they will use. Its counterpart is the Overdrive Meter.
The Rage Meter has 3 levels: Normal, Rage, and Max. The demons and bosses will use more powerful skills when their rage begins to increase. Like with the Overdrive Meter, this gauge fills as the demons get hit, but it also increases by a certain amount when they lose one of their comrades; when that happens, the increase becomes permanent and will become the base when it drops back down. Due to this aspect, when more of their allies are defeated, their rage will build up faster because the gauge essentially becomes shortened. This allows for them to more frequently use their stronger Rage Mode skills, as well as causing the Meter to fill up to Max faster.
When the Rage Meter peaks at Max, the next demon able to move will unleash their Break Out! skill, which is essentially their equivalent to a Lv. 3 Over Skill. Much like a Lv. 3 Overskill, the Meter will reduce back to its base level (which may not be completely empty, as mentioned before). The Break Out! varies with the demon's class; all non-generic bosses and the Accursed have unique Break Out! attacks.
In Yggdra Union, the Rage Meter now rises gradually during a clash. Once it reaches the Rage level, the enemy's attacks become slightly stronger. In addition, if their class has one, they will channel their element, making their attacks inflict elemental damage on the opponent. Once it rises to Max, the enemy will inflict maximum damage in the clash, but their element (if applicable) will nullify. In the Max stage, the enemy will use the Skill of the card they wield as soon as possible if they meet the criteria for it. If the Card's skill is class/character specific, then they will remain in Max mode for the rest of the fight until either side falls.
One advantage that the enemies have is that the Ace requirement for a Card does not apply to them. Also, during Unions, the state of the Rage Meter carries over to the next party if the previous one was defeated; conversely, if the player's unit lost the clash, then the Rage Meter resets itself. The Meter will also reset if the enemy uses their skill.
Returning in Knights in the Nightmare, this time the Rage Meter serves as a timer for when the enemy(s) will use their next attack. When the icon of the enemy reaches the peak of the gauge, they will perform one of their attacks; special actions do not apply to this and will be used by the enemies under given circumstances. For boss characters such as Jamie, the rate at which they can attack can be reduced by giving them their corresponding Key Item.