How do I unlock the second overdrive for Valefor in FFX?

Answer 1

At any point during the game go to the items shop in Besaid. Talk to the person there and she will say her dog found something. After you're done talking to her go to the house on the right side near the temple and you will find the dog inside. Press 'X' near it to get Valefor's second overdrive - Energy Blast.