There are two types of common EX Skills that can be equipped by any character: Buff EX Skills and Debuff EX Skills. Buff EX Skills enhance your stats and/or your allies’ stats, or provide extra utility such as teleportation and the ability to cure debuffs. Debuff EX Skills render your enemies weakened with negative afflictions, such as lowered defense and drained Bravery.
Common EX Skills may heavily dictate and influence your playstyle and your overall effectiveness on a team, so consider your EX Skills carefully. You can customize each character’s EX Skill Sets within the Character Customization interface, with each character having up to 10 separate loadouts (enough for essentially any situation and team composition you come across).
Note that the summons Ramuh and Leviathan affect certain common EX Skills, boosting the speed at which you gain EX Skill gauge from attacks and increasing buff/debuff duration. Ramuh enhances buff skills, while Leviathan enhances debuff skills.