|Jafar says: Read my lips and come to grips with the reality!|
Hooleer first appeared as a figure amongst a number of Gillian-class Hollows in a Negacion after Aizen revealed himself as a traitor, alongside Gin Ichimaru and Kaname Tōsen. They were brought to the portal via Casa Negacion and the Gillians took them into their arms as Hooleer looked on, with only its eye visible. Some time after this, after Aizen's forces had engaged the forces of the Soul Society in the fake version of Karakura Town,
Hooleer arrived during the battle, accompanied by Wonderweiss Margela. The creature used its breath to extinguish the fire prison generated by Captain-Commander Yamamoto keeping Aizen, Gin, and Tosen subdued, while Wonderweiss joined the fight. Eventually the Arrancar let loose his battle-cry, which prompted Hooleer to start expelling Gillians out of its mouth to deal with the arrival of the Visoreds.
However they slew the Gillians and one of them, Mashiro, attacked and killed the creature with a kick, while Wonderweiss looked on in shock.
Hooleer's most notable ability is its power to create a large amount of Gillian-class Menos Hollows, in the form of black liquid and masks that separate into individual Hollows. It also has very powerful breath, as seen when its breath dispelled the fire prison created by Yamamoto, who said it smells of death.
- Hooleer seems to have a connection to Gillians and appears to be some sort of Hollow, but since it doesn't have a mask, this is debatable. Some fans theorize that it is some sort of experiment created by Aizen from Gillians, perhaps by fusing them together into one being.
- Hooleer has Gillian Hollow Masks on its fingers so that they resemble fingernails.