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, with a white mask and long, light colored hair. It has two long forward-pointing horns protruding from the sides of its head and its Hollow hole is located in its upper chest but is filled in. It has two blades in place of its lower arms.

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

History
Some time ago, White was created as part of Aizen's research. It killed a number of Shinigami in Naruki City, prompting the arrival of Captain Isshin Shiba to investigate. When Isshin tried to lure it out by blasting large amounts of Reiatsu, White attacked two Shinigami that were sheltering nearby before confronting Isshin. It fought with Isshin until another Shinigami approached. White turned its attention to the new arrival and fired a Cero at him before resuming its battle with Isshin. As the captain released his Shikai, he was cut down by Aizen. The Hollow then continued to exchange blows with the injured Isshin, eventually losing one of its arms. Soon afterwards, Masaki Kurosaki attacked it. White avoided her attacks while charging towards her and bit her shoulder. She promptly fired an arrow at point-blank range, straight through its head. Being severely wounded by this, White begins to self-destruct, forcing Isshin to shield Masaki.

Powers and Abilities
Immense Speed: White is very fast, able to quickly sneak behind to unseated Shinigami and decapitate them before they can even realize his presence. Against a captain and Shunpo expert like Isshin Shiba, White's speed was able to keep the captain pressured.

Enhanced Strength: When Isshin Shiba had White seemingly pinned down by standing on one of his blades, White was able to launch Isshin into the sky with his pinned arm, destroying much of the ground around them.

Enhanced Endurance: Even after losing one of is arms, White was still able to pressure Isshin in battle (albeit Isshin was already injured by Aizen's sneak attack).

Enhanced Durability: His bladed arms repeatedly withstood impacting against Engetsu without showing any signs of damage until Isshin released his Zanpakutō.

Immense Spiritual Power: Has displayed reiatsu powerful enough to visibly intimidate Isshin Kurosaki, a Shinigami captain. According to Aizen, White should easily be able to defeat any Shinigami captain armed with Shikai.

Expert Hand-to-hand Combatant: Using the bladed appendages on the ends of his arms, he was able to hold his own against Isshin Kurosaki, a known Zanjutsu Master. Cero: White can use Cero (despite Isshin coming to the conclusion that he is not a Menos), which is powerful enough to obliterate an average shinigami without a trace. Even a captain-level shinigami, like Isshin Kurosaki, was afraid of getting hit by it.

Self-Destruct Mechanism:If wounded severely, White is capable of self-destructing. In doing so, first the matter in its Hollow hole expands, then a massive explosion follows, capable of heavily wounding a captain.

Trivia

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