Espada #09, the last of the original espada and the only Gilian, he can devour and copy the abilities of other hollow. One of the 33,650 + hollow he devoured was Metastacia, the one that took over Kaien Shiba’s body, assistant captain of the 13th squad in the seireitei. He used this form (Kaien Shiba) to attempt to get Rukia Kuchiki to kill her friends to atone for killing Kaien. However, Rukia just couldn’t accept that and knew that Aaroniero wasn’t Kaien and attacked. After revealing his true form (Glotonería) he stabbed Rukia with Kaien’s Nejibana (捩花), when she was stabbed she felt guilty for not being able to save Kaien. But she remembers that she has Kaien’s heart. Rukia kills him by impaling one of his two heads and shattering the “stasis tank” that his heads were kept in.

Source: Bleach official character books II