Ifrit is the second Aeon that Yuna receives and is a reoccurring summon in the Final Fantasy series. She receives it after you complete the Kilika Cloister of Trials. Ifrit can do regular attacks as well as an attack called Meteor Strike.
Note: Both Meteor Strike and Hellfire are fire-based attacks. Make sure that the enemy you are facing is not either resistant to fire, immune to fire or able to absorb fire magic or it will heal HP.