The Agility stat in Persona 3/Reload, Persona 4/Golden, and Persona 5/Royal prioritizes each party member's turn. Naturally, the one with the highest Agility will usually go first, the lowest goes last. In battle, the enemy's Agility is also calculated. If striking first for a chance to Down enemies is a necessity for combat, then it's a good idea to bring along party members that have above average speed.