"Ryuka isn't at home right now.... please leave a message after your death!"
Hollow Ryuka (虚 (ホロウ) 一護, Horō Ryuka), commonly known as Keiba (一護 の 內面 (なぃめん) の 虚 (ホロウ), Ryuka no naimen no Horō ), is a powerful entity who was created inside Ryuka's subconsciousness, out of Ryuka's hatred and grief.
Keiba's appearance resembles that of Ryuka's. The only obvious differences are his clothing, which consist of a white jacket, sash, and hakama, and his skin, which is also a greyish-white. His eyes and hair, however, are relatively the same as Ryuka's.
Inside of Ryuka, Keiba takes a human-like form that is exactly the same as Ryuka's own in shape. However, in this form Keiba has a white-gray skin tone, grayish-blue hair, and wears clothes different from his host's uniform.
Keiba takes this form during his fight with Ahatake. His hair turns pure white, his skin is gray, and his clothing gains a black color, mixed slightly with red gloves.
While in this form, half of Ryuka's Hollow Mask forms over the face of Ryuka in the real world. When this occurs, Keiba can take over Ryuka's body until Ryuka chooses to regain control. This is not so much a form as it is possession of a body, however.
When Ryuka witnessed first-hand the slaughter of his parents by Itachi Uchiha, the overwhelming amount of emotions led to the result of the birth of Keiba. At the time, Keiba was in an immature state, similar to a baby. As Ryuka grew, learning more about the world and his surroundings, Keiba was able to pick up on that knowledge as well, advancing right along with Ryuka.
Eventually, he would grow to become stronger than even Ryuka himself. He even attempted to gain control of Ruyka's soul several times. The first was immediately after Kyashi had rescued Ryuka from Echo's wrath. However, Kyashi was able to surpress him before he could do any major damage. The second time was when he attempted to kill Lynterria during a reunion between Kyashi and Sachi. Nearly getting killed, Lynterria was able to aid Ryuka to force Keiba back using a sealing arrow.
The third and final attempt was when Keiba confronted Ryuka himself, accompanied by Kyashi, Ahatake Kurosaki, Akiko Kurosaki, and a newly found Sachi Urami. He was angered by Keiba putting his comrades in danger. Keiba, also in an angered state, reveals that he had felt like a slave after Kyashi had taught Ryuka how to control his power. This relevance touched Ryuka. After a short fight, Ryuka defeated Keiba, who accepts the fact that his effort for control is worthless. He surrendered his power to Ryuka, officially claiming him as the rightful owner of his soul.
Keiba eventually gets to even terms with both Ryuka and Ahatake, accepting them as no longer his enemies, but possible allies. He even sees Ahatake as his very own sparring partner, after Ahatake challenges him to their very own fight. Once the fight started, Keiba, due to the huge power difference between him and Ryuka, gained the advantage.
Powers and Abilites
Keiba possesses a number of abilities of both from his Hollow heritage and Ryuka's own abilities. He is able to use all of them while in one of his three forms. He is able to effectively use all of these abilities, showing a sense of high mastership.
Gemelos Sonido: A faster version of Sonido that allows Keiba to move at a speed so fast, he actually leaves behind after-images of himself.
Instant Regeneration: Any damage sustained is instantly repaired, making Keiba nearly impossible to kill.
Bala (虚弾（バラ）, Bara; Spanish for "bullet") is an arrancar alternative to the cero. The technique hardens the user's spiritual pressure and fires it like a bullet from their fist. It is seriously weaker than a normal cero, it moves about twenty times faster and can be fired at a much higher rate, but the damage is much the same.
Cero (虚閃（セロ）, Sero; Spanish for "zero") are high-powered energy blasts that can be fired from various body parts, such as mouth, tongue, hand or fingers. Only Menos, Arrancar, and the Vizards have so far been shown to use cero attacks, and it appears that stronger entities can use it more efficiently. Various arrancar and hollow characters display variants of the technique, such as the Espada-exclusive gran rey cero (王虚の閃光（グラン・レイ・ セロ）, guran rei sero; Japanese for "royal hollow flash," Spanish for "great king zero")
Garganta (黒腔（ガルガンタ）, Garuganta; Spanish for "throat") is how arrancar and menos class hollows move to and from Hueco Mundo. Kaname Tōsen and Kisuke Urahara have also demonstrated the ability to use the technique. It literally tears open the dimensional fabric separating the worlds, revealing a tunnel of whirling, torrential energy that must be focused and solidified to create a discernible pathway.
Hierro (鋼皮（イエロ）, Hiero; Spanish for "iron") refers to the hardened skin of the arrancar, which is a result of their compressed spiritual power. While their skin is strong enough to block even released zanpakutō bare-handed, it is by no means impenetrable. Stronger arrancar have proportionally stronger skin.
Negación (反膜（ネガシオン）, Spanish for "negation") fields are used to rescue fellow hollows. Only the gillian-class menos have shown the ability to use it. Negación fields surround a target in a square beam of light that isolates the target from the dimension they are currently in, making it impossible to harm them. The field then pulls the target towards the menos that created the field. Espada have a special cube-shaped device called Caja Negación (反膜の匪（カハ・ネガシオン）, Spanish for "Negation Box") meant to simulate negación fields, which are used to punish their subordinates if necessary. The cube, when used on a subject, traps them in an alternate dimension from which there is no escape; however, this assumes it's being used as intended. More powerful Arrancar, namely the Espada themselves, can escape given enough time.
Pesquisa (探査回路（ペスキス）, Pesukisa; Spanish for "inquiry") is the Arrancar equivalent of the Shinigami ability to sense Spiritual Pressure. It functions similar to a sonar.
Originally, Keiba was believed to be the Keibatsu's inner spirit. However, after Ryuka's final fight for control of his own soul, it is revealed that Ryuka's actual inner spirit was a young boy, having been surpressed by Ryuka's demonic power and broken free. The boy possibly has stronger power than Keiba himself, as shown in Ryuka's fight against Otonami. Ryuka, fighting together with his inner spirit, was able to repel and destroy the offending blast. The power was so great, it took Otonami nearly all of his strength in order to negate it.
As with Hollow Seireitou and Kyuubi's existance inside Seireitou, Keiba and the boy serve as the source of Ryuka's power. The boy is the ruling power over Keiba, however, due to him having much of the control of Ryuka's soul.
Inside Ryuka, Keiba commonly wields his own version of Keibatsu as well, having the same abilities as Ryuka himself.
Initially, Keiba was a figure of malevolence. He was often brutal, sadistic, sarcastic, and arrogant towards other people. He believed often that Ryuka was weaker than he was, and despised him for his control over him. He never hesitated to commit violent and heartless acts against Ryuka and his closest comrades whenever he was in control of Ryuka's body. However, he is protective of Ryuka, due to the fact that if Ryuka dies, so would he.
Over time, after his defeat at Ryuka's hands, Keiba no longer harbors those feelings of hatred towards Ryuka, having gained a mutual respect towards him, along with Ahatake. A sibling relationship side can also be seen between him and Ryuka; when Ryuka is about to leave the mindscape, he vowed to assume control of Ryuka's body if he ever was in danger again. He is seemingly at odds with Otonami Sokudo, however, due to his dark and arrogant nature.