Obtained: During Chapter 6, speak with Iori who is standing inside the Cosmic Egg.
After you speak with him, he’ll disappear. Head to Shibuya Station to find him near the statue. Speak with him again to learn the Mirage is in Illusory Dolhr. Head into the Idolasphere and teleport to Altitude: 54m. Once there, head southwest on the island to find the Mirage near a ladder leading down. Speak with her then head back to Iori in Shibuya. Speak with him to complete the quest.
Reward: Def Increase x2