Ep. 155: Rukia fights back against Kaien, exposing his fear of sunlight and binding him with Kido.

155

Rukia struggles against Kaien, who gains the upper hand using his zanpakutou's shikai. After disarming her, Rukia attempts a kidou spell, inadvertently revealing that Kaien fears sunlight. Using this advantage, she binds him with kidou and blasts a hole in the wall. In the sunlight, Kaien's face melts away, revealing a glass case containing two masks. The creature reintroduces himself as Aaroniero and, after moving back into the shade, assumes Kaien's appearance again. He explains that he absorbed Kaien's remains upon arriving in Hueco Mundo. Aaroniero then releases his zanpakutou, transforming his lower body into a giant, octopus-like form. This event occurs during the Arrancar: The Fierce Fight arc, specifically episode 4 of 16.