Hueco Mundo

Aaroniero can be seen in the conference room, discussing with his fellow espadas what to do with the intruders. He later meets up with Rukia outside of his chamber and tells her to follow him. She follows him into a dark room, there he takes off his mask. Underneath his mask is the face of Kaien Shiba. Aaroniero tries to trick Rukia in to thinking he's Kaien Shiba, but his behaviour gave him away. 
 

Rukia attacks Aaroniero and pins him down. She had noticed before that Aaroniero didn't want to be exposed to light. She blasts a hole through a wall and unveils Aaroniero's true face, a glass container with two floating heads. Aaroniero then explains why he has Kaien's appearance, memories and Zanpakutou. Aaroniero is the only gillian class hollow in the Espada and that he is a hollow eater, by eating fellow hollows he becomes stronger and gains their ability.
 
When Kaien was still alive he used to be the mentor of Rukia and she looked up towards him. One night Kaien went to kill a Hollow, after the hollow had killed his wife. Kaien and the hollow fought, but he didn't know the true ability of the hollow. It had the ability to merge with other organisms. It took control over Kaien. After a while Rukia and her squad captain Ukitake appeared to stop the creature. Rukia had the battle Kaien as the captain was too ill to fight. She was very reluctant, but in the end managed to kill him out of self defense. 
 
When Kaien died the hollow/Kaien creature returned to Hueco Mundo, the hollow was an experiment of Aizen. Back in Heuco Mundo It was eaten by Aaroniero thus gaining the abilities and memories of Kaien. During his fight with Rukia Aaroniero points out that he is using the spirit body of Kaien Shiba and all his experiences and memories are in still intact. Aaroniero was using them to impersonate Kaien and manipulate Rukia in turn. After hearing this Rukia can't brin

g herself to attack Aaroniero, for she cannot bring herself to kill Kaien again. 
Aaroniero uses this oppurtunity to attack Rukia with Kaien's Zanpakutou Nejibana. He impales her and lifts her up. As Rukia is mortally wounded she thinks back about Kaien and remembers his teachings. Motivated by her memories of Kaien, she uses the last of her strength to perform a technique used by her zanpakutou, impaling Aaroniero through one of his facse and shattering the container. The  face that has been left over dies screaming in agony.
  

Chapter Hell

After his demise, Aaroniero was seen falling into an unknown place together with fellow Espada, Szayel Aporro Granz. Both are confused to where they are. They try to comprehend what happened to them and how to get out of their location. The discussion escalates and Aarroniero tries to attack Aporro. But before they can attack each other they are stopped by a mysterious figure, calling himself Shuren. Shuren informs the duo that hey have died and are currently in Hell. Aporro demands Shuren to let him go, Shuren responds by summoning three of his underlings. Before Aporro and Shuren come to blows, Aaroniero releases Glotoneria and attacks Shuren and his group. Aaroniero heads for the obese member of the group, called Taikon, but is easily defeated. After Aporro also falls at the hand of the group, Shuren ponders. Saying that beings of Aporro's and Aaroniero's level could never open 'that'.

Personality

 
Aaroniero is deceving and cruel, shown when he uses Kaien's appearance to try and trick Rukia into killing her friends. He is also very greedy, constantly wanting to evolve and become stronger by eating other Hollows. Fitting as his aspect of Death is Greed.
Somewhat cruel and sadistic, Aaroniero enjoys toying with his opponents.  Interestingly, his two heads appear to have a degree of individuality, since they refer to themselves in plural when exposed, and each reacted differently to the idea of death.
 

Powers and Abilities

Aaroniero Arruruerie is the 9th strongest Espada, he's techniqually the weakest Espada when Yammy releases his Zanpakutou. Like all Espada Aaroniero posseses the standard hollow powers such as Sonido, Hierro, Cero etc. Aaroniero also has the unique abilty to eat hollows, thereby gaining their powers, memories and experiences. It hasn't been explained if this ability is only works on eating hollows. As has also been able to use Kaien Shiba's Zanpakutou, Nejibana.

• Cero: Like all arrancars Aaroniero posseses a cero, but the color is unkown. 
  
• Sonido: Master During his battle with Rukia it is shown how fast at sonido he is, passing by her without Rukia noticing him.
• Garganta: The way how all arrancars travel to places. 
  
• Gran Rey Cero: An exclusive cero that all Espada posses.  
  
• Shapeshifting: Aaroniero has shown the power to shafeshift his face, to that of Kaien Shiba. 
  
• Absorption: Aaroniero main ability is to eat dead hollows to obtain their powers.

Zanpakutou

 
Glotonería ( Spanish for "Gluttony," Japanese for "Eating Hollow")  It takes the form of a worm with sharp teeth and tentacles, which he hides underneath his

sleeve.  
 
Resurrection: He activates it with the command Devour, it's name is Glotoneria. After releasing his Zanpakutou his whole lower body becomes a giant, smiling, multi-faced sort of Octopus. While in this form he can use all the powers of his devoured victims. Aside from having any ability of all hollows that he has consumed he still posseses Kaein's shinkai Nejibana, making jim very lethal.  
 

Nejibana:   Because of the absorption of hollow Mestacia, Aaroniero obtained Kaien's look and his Zanpaktou's shinkai Nejibana. a Trident based weapon with incredible reach, and water based attack, it is a very strong shinkai mainly a melee type zanpaktou.    

  

Aspect of Death

Like all Espada, Aaroniero represents and aspect of death, which are the 10 reasons human die, Aaroniero's aspect is Greed. This aspect of death is based on Aaroniero's power of Glotoneria, which is the many devouring of many dead hollows which have given him his rank as an Espada since their foundation, each time he absorbs more hollow and gains more power, he evolves more, he is the only Espada to evolve, and is the only one who is a Gillian, his thirst for evolution makes him more greedy to eat more hollow to obtain more power.