Each human character in Bleach consists of a body animated by his/her spirit identical in appearance. In the rare event that the spirit is separated from the living body, a linking structure known as the chain of fate (inga no kusari) attaches the spirit to its body thus allowing it to return to it. When the chain is broken by force or the human dies, the spirit cannot return to its body and becomes what is known as a plus (whole in English dub). If the person has many regrets or has a strong emotional attachment to a certain person, place, or thing, he/she will be connected to that by the chain of fate. At some point, the person's emotions will begin to corrupt and consume his/her soul, thus triggering the transformation into a hollow. Furthermore, a completely corroded chain of fate will also trigger the transformation into a hollow. However, as demonstrated by Ichigo, it is possible for a soul to avoid completely transforming into a hollow by unleashing his/her shinigami powers, instead becoming a visored.
When a plus transforms into a hollow, his/her soul initially disintegrates and then reforms as monster-like figure with a big hole in its body, usually the chest. A skull-like mask then forms on its head, thus completing the transformation. In unusual circumstances, once again demonstrated by Ichigo, the mask forms first while the body remains intact, showing the plus' resistance to the hollow transformation. A hollow loses all sense of reason and acts purely on the instinct to kill and consume the souls of people, usually starting with the ones who were closest to them. They are particularly attracted to souls with high levels of spiritual power; shinigami are a rare delicacy among hollows. Those with an unusually voracious appetite for other souls will attack and consume other hollows, eventually evolving into menos.
Menos (meaning "minus" or "less" in Spanish) is the result of hundreds of hollows compiling together, forming a larger, more powerful hollow entity. The menos have three stages of evolution which progressively have greater strength, speed, and intelligence. Menos reside in a dark area of Hueco Mundo called the Forest of Menos.
The first stage in a menos' evolution is called gillian (Japanese for "lowest of the hollows"), colloquially known as menos grande. Gillians are massive and powerful, but lack intelligence and individuality being the product of hundreds of hollows fused together. Though they are the weakest of the three types of menos, the appearance of even just one gillian in any setting outside of Hueco Mundo is cause for a state of emergency, in which case senior officers are usually dispatched to handle them. Nevertheless, the emergence of gillians, or any menos for that matter, on earth or in Soul Society is extremely uncommon. A hollow who manages to retain its individuality when becoming a menos will begin to attack and devour other gillians, thus increasing its power and triggering its second evolution.
The second stage of a menos' evolution is called Adjuchas (Japanese for "intermediate great hollow"). Adjuchas are the result of hollows who have continued to consume other hollows as gillians, thus causing their second evolution. It is estimated that hollows must consume another thousand or so hollows before evolving again. Adjuchas are much smaller in size than gillians, however much more powerful and intelligent, and therefore far more dangerous. They often act as leaders of groups of gillians and lesser hollows. The power these hollows possess are said to be comparable to a shinigami lieutenant or even higher in some cases. Nevertheless, adjuchas are smaller in number than gillians, so encountering one even in the Forest of Menos is rather uncommon. Should an adjuchas stop eating hollows for an extended period of time, it will lose its individuality and its evolution will reverse, thus reverting it back into a regular gillian with animalistic instincts, however permanently. Moreover, if any portion of an adjuchas' body is lost to or eaten by another hollow, its evolution will cease regardless of how many more hollows it consumes.
Vasto Lorde 最上大虚 (ヴァストローデ), vasuto rōde; Japanese for "Highest Great Hollow") is the third and final stage that a Menos class hollow can reach. They are the smallest out of all the other classes of hollows usually equating to the same size as that of a human. Because the level of Vasto Lorde is extremely difficult to reach, they are rare to come across. It is said that they are so scare one could count the number of them throughout Hueco Mundo with only the fingers on your hands. It is also said that they combat abilities surpass that of the captains of the Gotei 13.
Apart from their ability to cannibalise other hollow, the exact requirement for a hollow to reach that stage of evolution is unknown. Captain Tōshirō Hitsugaya stated that if Sōsuke Aizen were the gather more than 10 Arrancar of Vasto Lorde level, Soul Society would be doomed. Out of the Espada, Tia Hallibel is the only one confirmed to be on Vasto Lorde level.
Arrancars are hollows who have acquired soul reaper powers through a process in which their hollow masks are almost completely removed. They appear almost entirely human and possess intelligence far superior to that of the normal hollow. The term arrancar (Japanese for "broken mask") is a Spanish verb meaning "to tear off," referring to the process in which a hollow becomes an arrancar by tearing his/her mask off. Arrancars become the primary antagonists in the second part of the Bleach anime and manga series as they are commanded by Sosuke Aizen.
When Aizen began creating the arrancars, he established a hierarchy similar to that of the soul reapers which dictates their positions and occupations. However, the style is much different and somewhat unorthodox. In the lower ranks of the arrancars are the Números (Spanish for "numbers"), who are named so because they are numbered in order by when they are born. Números, though generally considered weak, have proven to be somewhat of a match for soul reaper lieutenants and in few cases captains.
Fracción (Spanish for "fraction," Japanese for "subordinate officers") are Números who serve as subordinates of a specific Espada. Fracción are often extremely servile and loyal to their respective Espada superiors, and will diligently follow any commands given to them. The number of fracción an Espada has varies from one to as many as over a dozen. However, some Espada choose not to have any fracción under their control.
In the top ranks of arrancars are the Espada (Spanish for "sword," Japanese for "ten blades"). They are the distinguished elite of the arrancars and are numbered from one to ten (or zero to nine) according to their power, unlike Números who are numbered by the order in which they are born. Members of the Espada have their number tattood on their body (the location varies for each member) signifying their rank and power. Espada are allowed to choose any Números to be their fracción. Each member has power comparable to or higher than that of soul reaper captains. Therefore, some restrictions have been placed on the use of their power and abilities in Las Noches, especially on those ranked four and above.
Main article: Coyote Starrk
Coyote is the Primera Espada ("primary" or "first" Espada) in Aizen's army of arrancars. Stark is generally easy going and often lazy, and uses up most of his time sleeping or lying down. He dislikes fighting primarily because of his slothful attitude, though he will if commanded to do so by Aizen. His number "1" tattoo is located on his left hand, which is usually covered by a white glove.
Main article: Barragan Luisenbarn
Barragan is the Segunda Espada ("second" Espada) in Aizen's army of arrancars. Barragan's appearance and command presence (as well as taking advantage of Stark's nonchalance) have at times lead others to mistake him as the first Espada. His attitude is generally grouchy and he's often cynical. The remainder of his hollow mask forms a five-pointed crown. Barragan has six fracción under his control all of which are male.
Neliel Tu Oderschvank
Main article: Neliel Tu Oderschvank
Neliel, shortened Nel Tu or simply Nel, is the former Tres Espada ("third" Espada) which is signified by a large number "3" tattoo on her back. Nel differed from her fellow Espada for her pacifist ideals, which led her into conflict with Nnoitra, who was then the eight Espada. After being ambushed by Nnoitra and Szayel Aporro Granz, she suffered a debilitating injury to her head which caused her to transform into a child. She was then exiled from the Espada and Las Noches, along with her two fracción, Pesche Guatiche and Dondochakka Bilstin. As a child, Nel is very rambunctious and affectionate, and she speaks with various speech impediments.
Main article: Tia Hallibel
Tia Hallibel is the current Tres Espada ("third" Espada) which is signified by a small number "3" tattoo on her right breast. She is presently the only female Espada since her predecessor Neliel Tu Oderschvank. Keeping half of her face concealed, Hallibel maintains a mysterious aura about herself, which is also exemplified in her personality. She is very composed and rarely speaks, and when she does it is often just to teach a moral or principle.
Main article: Ulquiorra Cifer
Ulquiorra is the Cuarta Espada ("fourth" Espada) and one of the first Espada to debut in the Bleach anime and manga series. The remainder of his hollow mask resembles a helmet-like appendage with a horn, and his number "4" tattoo is located on the left side of his chest. He maintains a stoic, melancholic appearance which reflects his overall personality. Ulquiorra is anti-social, cynical, intelligent, and misanthropic, periodically ridiculing humans for their camaraderie and their emotional attachments to each other. Though he is sometimes hasty to strike his enemies, Ulquiorra will often refrain from using violence unless ordered or prompted to do so.
Main article: Nnoitra Jiruga
Nnoitra is the Quinta Espada ("fifth" Espada), signified by a number "5" tattoo on his tongue. He is tall and wears an eye-patch which covers the remainder of his hollow mask and his hollow hole which is located where his left eye would normally be. He is very cocky and claims to have the strongest hierro of any Espada in the past and present, which enables him to repel attacks even from sharp weapons by his skin alone. Nnoitra is a firm believer in male dominance and hates the idea of women outranking or being stronger than men. Therefore, he has made it his goal to give women in higher positions their "deserved" inferior rank. This mindset has led him to attack Nel, who frequently defeated him in battle although never killing him.
Main article: Grimmjow Jaegerjaques
Grimmjow is the Sexta Espada ("sixth" Espada) which is signified by a number "6" tattoo on the right side of his back. He is the first arrancar revealed to be an Espada in the Bleach anime and manga series, though not the first to debut. The remnants of his hollow mask resembles a jawbone with teeth, which is located on his right cheek. Grimmjow is the most outgoing Espada, though his overall personality and attitude is often disrespectful. Like several others, Grimmjow lusts after battle, pursuing anyone who has an abnormally high spirit force. He is very confident in his abilities and believes himself to be as strong, if not stronger than his Espada rival Ulquiorra. As a result, he has developed an inferiority complex, loathing anyone who looks down on him.
Main article: Luppi
Luppi is Grimmjow's temporary replacement as the Sexta Espada, signified by the number "6" tattoo on his right hip. He has a youthful and feminine appearance, and tends to belittle his opponents and arrancar lower in rank. Superficially, Luppi seems to be the youngest Espada, although in actuality, arrancar do not age, therefore their semblance is irrelevant in determining such things. Luppi's term as the sixth Espada is ephemeral, as Grimmjow's arm is restored not long after his promotion, which Grimmjow used to impale and annihilate Luppi.
Main article: Zommari Leroux
Zommari Leroux is the Séptima Espada ("seventh" Espada) in Sōsuke Aizen's army of arrancars. The remainder of his hollow mask forms a necklace of teeth as well as a ridge of spikes running down the center of his head. He is soft-spoken, modest, and holds Aizen in very high regard while loathing shinigami for exercising unrighteous judgment upon hollows. Zommari has the self-proclaimed fastest sonido of the Espada, called Gemelos Sonido (Spanish and Japanese for "sound twins"), which creates up to five replications of himself.
Szayel Aporro Granz
Main article: Szayel Aporro Granz
Szayel Aporro Granz is the Octava Espada ("eigth" Espada) and the younger brother of Yylfordt Granz, Grimmjow's fraccion. The remainder of his hollow mask forms rectangular framed glasses. He is a proud intellectual and scientist, meticulously analyzing subjects that interest him and organizing his plans down to the most minute detail. However, his research serves to satisfy a sadistic pleasure of using other people as dispensable test subjects. Szayel Aporro has the most fraccion under his control, which are modified to completely restore him when consumed, and can be recreated.
Main article: Aaroniero Arruruerie
Aaroniero is the Novena Espada ("ninth" Espada) and considered the weakest among the group. While he possesses a single body, Aaroniero has two small ball shaped heads which float inside a cylinder attached to his body. Each of these heads house different personalities although both of them refer to themselves as one person. Aaroniero is the only Gillian of the Espada thanks to his ability that allows him to absorb other hollows and their powers. Incidentally, one of the hollows he managed to absorb was Metastacia, who died while in possession of Shiba Kaien's body. As a result, Aaroniero was able to masquerade as Shiba as well as use his powers which almost allowed him to successfully manipulate Rukia Kuchiki.
Main article: Yammy Riyalgo
Yammy is the Décima Espada ("tenth" Espada), signified by a number "10" tattoo on his left shoulder. However, upon the release of his zanpakuto, his rank changes from a ten to a zero, thus making him the Cero Espada ("zero Espada"). Yammy is very tall and burly, and the remainder of his hollow mask forms a chin with teeth. Along with his large stature is an inflated ego, which has lead to a few mishaps that caused him a great deal of damage. Yammy also has issues controlling his anger, although this becomes his greatest strength while in resurreccion.
Hollows, like Shinigami, display several powers and abilities that can be used as a collective, and used by all. The more powerful the hollow the more access that have to collective powers with a larger destructive force or a more complicated nature, rather than relying on their own unique ability. Hollows tend to fuel their power by devouring souls the deceased and some have even displayed the power to extract souls from the still living. However lower level Hollow extract energy from the air in Reiryoku rich areas, such as Hueco Mundo.