The Nebula Armlet is a secret item found in Phoenotopia and possibly Phoenotopia Awakening. It allows Gale to focus her Ki more efficiently, thereby allowing her to charge up her attacks much more quickly than before.
The Nebula Armlet is located in the caves of Atai, which can be accessed by jumping into the water well in the middle of the city. From there, the player needs the Floaty Donut to swim across the water and enter the caves. The Bombs and Artifact are also a necessity here, as the caves contain Ancient Sentry Towers and the door to the room containing the Nebula Armlet is blocked off by stone blocks. When you get to the pit full of stone blocks, use the Bombs to clear away the blocks and reveal the doorway. The room inside is full of Zombots that you'll have to dispose of; once they're gone, you'll gain access to another part of the room which has a chest containing the Nebula Armlet.