In battle, your Overdrive meter charges to each character when something specific happens to them or the people around them. These are known as Overdrive Modes, and can be changed for any other modes during the game by accessing the Menu. To accquire a new Overdrive Mode, there are specific requirements that you have to do in battle or multiple battles to recieve.

Final Fantasy X Overdrive Modes
Mode	Description
Stoic	Charges when recieving damage
Warrior	Charges when damaging enemies
Comrade	Charges when allies take damage
Slayer	Charges when character defeats an enemy
Hero	Charges when character defeats stronger enemy
Ally	Charges on character's turn
Daredevil	Charges when character's HP is in a critical state
Tactician	Charges when character inflicts status changes
Avenger	Charges when a party member is downed
Sufferer	Charges when character is status changed
Victor	Charges when party wins a battle
Victim	Charges when charcter is attacked with a status attack
Coward	Charges when party flees from battles
Solo	Charges when character fights alone
Dancer	Charges when character evades attack
Healer	Charges whencharacter heals an allied party member
Rook	Charges when character blocks attack