Obtained: During Chapter 6 (after you enter the Cosmos Egg), speak with Urabe in Daiba Studio.
She’ll give you a hint that the item is in a watery channel within your memory. If you haven’t figured it out yet, you’ll need to head on over to the Bloom Palace Idolasphere. The item is located straight ahead on the very far side of the room near the First Relief. The easiest way to get to it is to teleport to the First Relief then take a right down the stairs. Keep heading straight and the item will be on the ground. Pick up the Treasure Key then head back to Urabe in Daiba Studio to give her the good news.
Reward: Kiddie Candy