Demeter's minigame, walking and successfully hitting an enemy will increase your agility.
Demeter's minigame will raise the agility only for Bell, the other two will raise the agility for everyone in your party.
Walking will give the same quality of Excelia, whereas successfully hitting an enemy will grant higher quality excelia the stronger the enemy is.
You can also increase the % of Excelia gained if you dine at the Hostess of Fertility once a day.