Valefor is the first Aeon that Yuna receives. She receives it very early on in the story before you even have an opportunity to include Yuna in a fiend battle after visiting the Besaid Cloister of Trials.
Aside from her regular attack, Valefor can also use an ability called Sonic Wings. This ability has the potential to cause the ‘Delay’ effect which pushes back the enemies turn (as long as they are not immune to this effect).
Overdrive: Energy Ray / Energy Blast
Note: Valefor starts off with the Energy Ray Overdrive. You can pick up the Energy Blast Overdrive by returning to Besaid Village and speaking with a young girl with a dog. Speak to the girl and the dog in order to obtain it, but be cautious, as once you reach the end of the game Besaid Village will be blocked off by Dark Valefor whom you must defeat in order to gain entry. Check out the Dark Aeons side quest section for more information.
Valefor will be able to break the 9,999 damage limit after you have obtained Nirvana and upgraded it using the Moon Crest.