One of the mechanics for battles in Dragon Quest XI is Pep. A character can enter a pep state during battles (seemingly at random, as there is no indication they will), which increases some stats and makes them stronger. It only lasts a few turns, though, with pep being removed. As you learn abilities on the skill trees, you will gain access to something called Pep Powers. These can range from attacks, as well as buffs, and can be trigger when only a single character is pepped up or multiple characters are. In order to unlock this, you simply need to use Pep Powers 25 times. It doesn’t matter if you use the same ones over and over, as it will still count.