Boost

The Boost mechanic is an empowerment mechanic originating in Etrian Odyssey and returning in Etrian Odyssey Untold: The Millennium Girl. It also gets reused in Persona Q: Shadow of the Labyrinth and Persona Q2: New Cinema Labyrinth but with a significantly different behavior.

Etrian Odyssey and Etrian Odyssey Untold: The Millennium Girl

Each party member comes with a Boost Gauge which caps out at 100. Each action they do fills the gauge by a random number between 5 to 15. Skills and equipment, like the Dark Hunter's Boost Up, can increase the increments and potentially grant them more than 15 Boost per action. If a party member dies, their current Boost level is halved while every other party member gains Boost equal to an action taken during their turn. At the end of combat, every living party member also gains Boost equal to an action taken. Resting at the inn will reset everyone's Boost to 0.

Once a party member's Boost Gauge full, they can opt to Boost their action for the turn. Normal attacks, when Boosted, do double damage, while other skills are treated as if they are five levels higher. Level 6 or higher skills, when Boosted, can go past the level 10 cap, maximizing their potency at level 15 (level 10 skill with Boost).

If an character acts multiple times in a single turn, like through the Gunner's Action Boost skill, all actions taken that turn will be Boosted.

Persona Q: Shadow of the Labyrinth and Persona Q2: New Cinema Labyrinth

In an approximation of the "One More" system of the Persona series, a character enters a Boost state if they strike an enemy's weakness or land a critical hit. While Boosted, the character's next action has increased action speed and their skill costs are reduced to 0, but Boost will expire immediately upon acting unless they trigger it again. If a character is hit by an attack (including if the attack was absorbed, but not if it was nullified) they lose the Boost status. If they lost their Boost status before taking their action, they must pay their skill's full cost (and it will fail if they cannot).

In Persona Q: Shadow of the Labyrinth, at the end of each turn, if enough characters are currently Boosted, there is a chance that the player can initiate an All-Out Attack to inflict a large amount of almighty damage to all enemies.

In Persona Q2: New Cinema Labyrinth, the All-Out Attack is not dependent on Boosted party members, and is available the moment all enemies are knocked down. Instead, Boost can be moved between party members using a Baton Pass function (at the cost of 2 Party Gauge charges). There are also physical skills that inflict increased damage if used while Boosted.