Hooleer

is a large entity that appears to be fighting alongside Sōsuke Aizen. ITs true nature remains unknown, but it seems to have a connection with Gillian-class Menos, suggesting that it is indeed a Hollow.

Character Outline
It has a bulbous body, long tufts of hair on its back, and a single, large eye. Its hair appears to be the same or a similar color to his skin. It also has the masks of Gillian-class Menos for fingernails. It had a brief appearance early in the series, but recently made its proper debut during Aizen's invasion of the Fake Karakura Town. It is arguably the largest creature seen in the series yet.

Soul Society arc
It first makes a short appearance as Sōsuke Aizen, Gin Ichimaru, and Kaname Tōsen are being extracted from Sōkyoku Hill via Negación. Its partially closed eye can be briefly seen behind the mass of Gillians present within the Garganta, but the rest of its body remains hidden.

Fake Karakura Town arc
Wonderweiss Margera appears during the Gotei 13's battle with the Espada, bringing Fūrā. It is evident that it dwarfs the other combatants in terms of size. While Wonderweiss attacks Jūshirō Ukitake, Fūrā uses its breath to extinguish the fire prison that is trapping Aizen, Ichimaru and Tōsen, thus freeing them. After Wonderweiss screams, it releases Gillians from his mouth to attack the Vizards. After the Vizards kill the last Gillian, Mashiro Kuna attacks it with her "Mashiro Super Kick", seemingly killing it, while Wonderweiss simply stares. Following Mashiro's attack, Ichimaru comments on how easily she took out Fūrā even though it was Wonderweiss's favourite.

Powers & Abilities
Breath: Fūrā's breath is powerful enough to put out the prison of fire made by Genryūsai Shigekuni Yamamoto's zanpakutō's ability, Jōkaku Enjō (though it must be said that they had already been burning for some time at this point). It is said that it "smells of death." Whether this means it has other more obscure effects remains to be seen.

Menos Manipulation: It is shown to be able to summon numerous Gillian Menos from its mouth. At first, it looks like a mass of black liquid, until the Gillian Menos separate into their individual forms.

Appearances in other Media
Fūrā appears as one of Aizen's special moves in Bleach: Blade Battlers 2nd, where it spits out Gillians to attack.

Trivia

 * Before Fūrā's real name was known, many fans referred to him as Superchunky because he was first fully introduced in the chapter titled "Superchunky from Hell".
 * Beyond having Gillian masks as fingernails, Fūrā does not appear to have a mask.
 * After Fūrā's death it is implied by Gin that there are more than one of its kind..