White

is an experimental Hollow used by Sōsuke Aizen many years prior to his defection from Soul Society. It was created based on a Shinigami's soul.

Appearance
White is an almost completely black Hollow. It has a white mask, and its light colored hair reaches down to the middle of its back. It has two long, forward-pointing horns protruding from the sides of its head, and its Hollow hole, located in its upper chest, is filled in. It has two blades in place of its forearms.

Personality
White exhibits a cold, calculating personality. Like most Hollows, it acts entirely on instinct, and always chooses the weakest victims in the battlefield. It did not respond when questioned by Isshin Kurosaki. During their fight, Isshin noted it felt like he was fighting something more akin to a Shinigami rather than a Hollow.

History
Some time ago, White was created from the souls of many Shinigami as part of Sōsuke Aizen's research. It killed a number of Shinigami in Naruki City, prompting 10th Division Captain Isshin Shiba to investigate. When Isshin tried to lure it out by exerting large amounts of Reiatsu, White attacked two nearby Shinigami before confronting Isshin.

As it fought with Isshin, another Shinigami approached. Turning its attention to the new arrival, White fired a Cero at him before resuming its battle with Isshin. After the captain released his Shikai and was attacked by Aizen, White, continuing to exchange blows with him, lost one of its arms. When Masaki Kurosaki attacked it, White, avoiding her attacks while charging towards her, bit her shoulder before she fired a Heilig Pfeil through its head. Having been severely wounded by this, White self-destructed, forcing Isshin to shield Masaki.

But despite its apparent demise, by biting Masaki and destroying itself, White had already chosen her as its target for Hollowfication, achieving its final "transfer" stage. Having entered her soul, the day after the battle it starts to Hollowfy her, putting her at risk of Soul Suicide. However, by utilizing a special Gigai invented by Kisuke Urahara, Isshin was able to link his soul to both Masaki's soul and White, suppressing her Hollowfication. White's influence would later be passed onto Ichigo Kurosaki, fusing with his Shinigami powers and becoming his Zanpakutō - Zangetsu.

Powers and Abilities
Immense Speed: White is extremely fast, sneaking behind two unseated Shinigami and decapitating them before they even registered its presence. White's speed allows it to keep up with and pressure a captain and Shunpo expert like Isshin Shiba.

Enhanced Strength: White possesses great strength. When Isshin pinned down its arm, White launched him into the sky, destroying much of the ground around them.

Enhanced Endurance: Even after losing its left arm, White remained able to pressure an injured Isshin in battle.

Enhanced Durability: White is a very durable fighter. Its bladed arms repeatedly withstood clashing with Engetsu without showing any signs of damage until Isshin released his Zanpakutō.

Immense Spiritual Power: White's Reiatsu is powerful enough to visibly intimidate Isshin Shiba, a Shinigami captain. According to Sōsuke Aizen, White should be able to easily defeat any Shinigami captain armed with Shikai.

Expert Hand-to-Hand Combatant: Using the bladed appendages on the ends of its arms, White held its own against Isshin Shiba, a known master swordsman.

Cero: Despite Isshin deducing it is not a Menos, White can fire a Cero, which is powerful enough to obliterate an average Shinigami without a trace. Even Isshin, a captain-level Shinigami, was afraid of getting hit by it.

Self-Destruct Mechanism: If White is severely wounded, the matter in its Hollow hole expands, followed by a massive explosion, which can heavily wound a captain.

Soul Infiltration: Much like Asauchi, White is composed of the souls of many Shinigami. Because of this, it is capable of entering the soul of an individual and fusing with their latent Shinigami powers. By doing so, the Hollow essentially becomes the victim's Zanpakutō. Though Aizen designed White for the purpose of Hollowfying Shinigami souls, it is also capable of entering those of Quincy.

Trivia

 * Sōsuke Aizen considers its name, White, to be ironic, for White has a black exterior but an "inner whiteness", due to its origins as a Shinigami's soul.