|Arrancar Rank||No. 9 (Espada)|
|First Appearance||Anime: Episode 153|
Manga: Chapter 267
In the majority of his debut, Aaroniero dons the face of Kaien Shiba, whose spirit particles he absorbed via devouring Metastacia. However, when all disguises are removed he is revealed to have two spherical heads both tattooed with the number nine that float about in a tall cylindrical glass dome attached firmly to his shoulders. Aaroniero is considerably tall and wears a mask when outdoor or exposed to light. He wears a dress garb that covers his entire body instead of the normal jacket and hakama common to arrancars. Moreover, he wears a frilly nobles' coat and two white gentleman's gloves.
Aaroniero's true personality is somewhat vague since for the majority of his appearance he's imitating Kaien. His two heads appear to have some individuality, referring to themselves in plural and occasionally have opposing views on certain subjects. The top head speaks in a low, deep voice while the bottom one speaks in a high pitched voice. Aaroniero is very manipulative, almost successfully convincing Rukia of his authenticity as Kaien. His status as an Espada has made him quite prideful, despite only being a gillian. He detests light and the outdoors, which is why he wears a mask and prefers dark rooms.
Aaroniero is the Novena Espada and considered to be the weakest Espada. Nevertheless, he has a few unique abilities that makes him quite useful.
Aaroniero began as a gillian, the weakest class of menos. It is assumed that he gained much of his power through his ability to absorb other hollows, totaling 33,650 by the time he became an arrancar. At some point, he ended up stealing the mind and body of Kaien Shiba, the former 13th Division Lieutenant and Rukia's superior. This was accomplished after he absorbed Metastacia, a hollow capable of absorbing other kinds of spirits, including soul reapers. Kaien had originally been sent to kill this hollow before it could absorb anymore hollow, but Kaien lost to him in the end. Rukia was forced to attack Kaien, hoping to kill the gluttonous hollow and save Kaien's soul from a terrible fate. However, this first attempt failed, and Aaroniero was later discovered by Aizen, who made him into an Arrancar using the Hōgyoku.
Aaroniero makes an appearance in the Espada meeting after Ichigo and company invade Hueco Mundo. He is mostly silent in this conversation. This is his first, albeit minor appearance.
He later appears after Rukia enters the sky dome of Las Noches. He guides Rukia to a nearby room, where he takes off his mask, donning the face of Kaien Shiba.