Spirits of Rage (info, skills, hints and etc.) *may include SPOILERS*

[Coin-coin le Canapin]

First, do the plugen quest.
Then you'll be able to summon the rage spirits in fight.
The rage increases when doing and receiving damages in a battle.
When you have enough rage, you can summon them.

The spirits gain experience when you're using their spell, and they'll be able to learn and upgrade new spells after a few fights.