While most souls move on to the Soul Society when they die, some remain in the Human World, unable to move on due to various reasons (i.e. protecting a location that bears significance to them, watching over loved ones, or carrying out a grudge). These souls (referred to as Pluses) cannot deal with being forgotten and if a Shinigami does not perform Soul Burial, which sends a soul to the Soul Society, they can become dangerously obsessed with what keeps them in the Human World.
Thus they become a Hollow, though the process can take months, even years, reflected by the length of the Chain of Fate that binds the soul to the Human World. It slowly corrodes as the soul’s ties to what keeps them there degrades. The process can be accelerated, via means such as being attacked by Hollows, being overcome with despair, or through certain spiritual abilities. When a soul is still becoming a Hollow, it becomes what is called a Demi-Hollow. They are bound by chains, usually to an area they consider their territory and are hard to sense by Shinigami, only appearing when someone steps into their territory.
When the chain decays completely, the soul becomes a Hollow with a hole forming their chest where the Chain of Fate was attached, showing their loss of their hearts, and a skeletal mask forming on their faces from the hearts that they have lost.
Hollows are driven by a never-ending desire to devour other souls to fill the void within themselves. While some may focus on the object of the obsession that kept them in the Human World, most Hollows simply try to devour as many souls as they can, preferably the ones with high spiritual power. Some of the more powerful and malevolent Hollows retain their intelligence, which makes them all the more dangerous.
However, some show the ability to express loyalty and while most are evil or at least mindless beasts, some can actually be non-malevolent creatures. Destroying a Hollow requires a spiritual means such as a Shinigami’s Zanpakuto, which purifies them and sends them to the Soul Society, or a Quincy’s bow, which utterly destroys the soul.
Hollows have a number of common powers, though most have their own unique abilities. They usually gain power by feeding on souls, but smaller ones can survive purely on condensed spiritual energy in places where it is concentrated like Hueco Mundo.
A few have demonstrated the ability to remove living souls from their bodies, often for the obsessions that kept them in the Human World. All Hollows apparently have an acidic touch, that leaves marks on Humans if they touch them, which seems to allow the Hollow’s spiritual energy to enter the person, causing them great pain. Hollows go to and from the human world via Garganta, where they tear apart the dimensional fabric separating Hueco Mundo from the Human World.
Hollows can also quickly regenerate from most wounds, excluding ones on their heads or vital organs. Menos Hollows have also demonstrated the ability to use Cero, a powerful energy blast.
Menos are a sub-class of Hollow and are more powerful than the common ones that are transformed Human souls. The Menos on the other hand, are the result of hundreds of Hollows fusing into one being, which is why they are much stronger. Thus only well-trained Shinigami teams are relied on to deal with them, but the Menos rarely leave Hueco Mundo unless they sense a large source of spiritual power or are spurred to by others. The Menos are divided into 3 groups, each with different levels of power, Gillians, Adjuchas, and Vasto Lordes.
The first and weakest class of Menos, Gillians are commonly considered to be foot soldiers. They all have the same appearance and are the most well-known Menos amongst Shinigami, sometimes referred to as Menos Grande. While they stand at least 2 to 6 stories tall, Gillians have the instincts of wild beasts and can easily be dispatched by a
Shinigami captain. They are able to use a Cero, Kumon, a spatial distortion that they use to appear in the Human World, and Negación, a field they can generate to rescue their comrades. Gillians also possess long tongues that can break other Hollows’ masks and while they do have hands with long sharp nails, they commonly keep them hidden in their cloaks. The process that creates a Gillian begins with certain Hollows, whose inner voids become so great that Human souls no longer satisfy them and they are driven to devour other Hollows. When enough of these cannibal Hollows, who are naturally attracted to each other, gather in one place, they begin trying to feed on one another, eventually fusing into a Gillian.
However, in some cases, if one of the Hollows has a stronger personality than the others, it will emerge in control of the Gillian, bearing a unique mask to show this. These Gillians continue to devour other Hollows, including its fellow Gillians, until it evolves to the next level of Adjuchas.
Adjuchas are the second-class of Menos and smaller than Gillians, as well as much fewer in number. However they are much more intelligent and powerful, able to challenge a captain-level Shinigami. Adjuchas are loners by nature, rarely joining into groups, and are often the ones who give orders to the Gillians. The Adjuchas are more akin to normal Hollows in that they can have a variety of appearances and unique individual powers. However for these Menos, devouring other Hollows to continue growing becomes a necessity. If an Adjuchas does not, its mind will be assimilated with the other Hollows comprising its being, causing it to regress back into a Gillian and lose its individual personality, thus making it unable to ever become an Adjuchas again. Also if any part of Adjuchas is eaten by another Hollow, it will become unable to continue evolving and seemingly not regress back into a Gillian. Besides this, some Adjuchas apparently can’t continue evolving to become a Vasto Lorde while others can, but it is unknown exactly why this is.
The last class of Menos, Vasto Lordes are also the most powerful. They are the smallest, roughly human in size, and are the fewest in number, to the point that one could count all the Vasto Lordes in existence with just their fingers. Vasto Lordes are said to be stronger than Shinigami captains and Toshiro Hitsugaya has stated that if Sōsuke Aizen were to recruit ten, the Soul Society would be doomed.
- Hollows, along with the Arrancars, have had a Spanish motif to them, such as through their classifications and the names of their abilities.
- London's equivalent of Hollows are Dragons.