Baseline Information

Hollows are the unlucky souls whom weren't or couldn't be saved by a shinigami. Their soul chain eroded from their chest, leaving their chest with a gaping hole in it. The hole is a metaphor for the hollow having no heart. Instead of a heart, the hollow has gained a white mask made of a bone-like substance. After transforming into a much larger, more fearsome beast, the soul has lost all reason and morals, becoming a savage killing machine whom devours souls and destroys everything around it. Due to their nature, they have no ranking system or hierarchy. Every hollow for themselves. Residing in Hueco Mundo, the hollows are capable of traveling to earth through two techniques, Kumon and Garganta.

Hollowfication Process and Evolution

Due to several different possible reasons resulting in the soul not becoming purified and sent to Soul Society, the soul will undergo a transformation into a Hollow. Hollows are large demonic creatures that feed on the souls of the living or dead in order to attain greater power. After transformation, a hollow will be transported to Hueco Mundo via an Kumon, a basic hollow skill. They are then free to reside there or move back to earth using the same method, although the technique will be controlled at this point.

Upon Returning to Earth

If a hollow decides to return to earth, they will most likely go after the person they loved the most when alive. This is because a hollow is a carnal beast ruled by it's instincts. They want that being so much that it becomes a hunger. If they succeed in their dastardly plan, that soul will be consumed. Afterwards, the hollow will begin destroying and devouring it around them, until it is either slain or returns to Hueco Mundo.

Upon choosing to stay in/returning to Hueco Mundo

If a hollow decides to stay in Hueco Mundo (or returns there at any point), their hunger will become focused on the other hollow around them. By centering this bloodlust on them, the hollow will begin feeding on one another. If a hollow feeds on enough other hollows, they will grow in power and attain new heights of existence.

Hollow Classification

Hollow have different classifications based on their general appearance, overall power level, and state of intelligence. The more hollows a Hollow devours, the higher up on the list he/she becomes. If a hollow has a piece of them devoured -no matter the size- by another hollow, they will never evolve from their current state. If a hollow stops devouring other hollows, they may actually regress back down the stages. Because of these two factors, some hollows ask for another to devour a piece of them to make sure that they remain at their current level. Although this was never stated in the manga and it is merely an observation, most hollows tend to typically be male. It appears that shinigami only want to save the pretty wimmenz.

Hollow

The most basic of hollow is generally simply called a "hollow". It retains a slight bit of intelligence, although a large amount of malice has been added to this. A basic hollow is not much of a threat to anyone with supernatural abilities, seeing as Ichigo and Uryu once had a contest to see whom could kill as many as possible. Even Karin is capable of killing a basic hollow. Usually, a basic hollow has a quadraped body structure, as well as a predator theme, such as a canine or feline. Having one or two basic abilities, the general hollow is surprisingly versatile.

Gillian

Sometimes, a hollow will begin eating a hollow that will begin eating it back. If enough of this happens at once, a feeding frenzy will begin, and an orgy of hollows eating each other will occur. When enough hollows join in on this, and one personality from the mass of hollows takes over, a gillian is truly born. Gillians are huge monster Hollows towering at least 2-6 stories tall, easily dwarfing even the largest standard Hollow. They all have a white mask with an elongated nose. Their bodies are covered from theirs head to their feet with a black covering reminiscent of a cloak. Around their neck is a row of thin white spikes. They have large, pointed feet wrapped in white bandages. They have large white hands with long, sharp, white nails, but they are commonly hidden within their cloak, rarely revealed even in combat. They also have long tongues capable of breaking a fellow Hollow's mask. (Appearance courtesy of bleach.wikia.org)

Adjuchas

Much smaller then a gillian, an adjuchas hollow has regained some of it's intelligence and rationality. More humanoid in appearance than a gillian, they are still quite animalistic. The role of an adjuchas is usually to lead the gillian into battle. Possessing a more advanced version of the general ability they had as a regular hollow (which was replaced in the gillian stage by Cero), as well as the racial abilities of an adjuchas hollow, they are capable of taking on captains of the gotei 6.

Vasto Lorde

Vasto lordes a very rare level of hollow. In order to become a vasto lorde, insane amounts of hollow must be consumed, as well as no piece of the hollow's body can be eaten. Because of these, as well as another possible factor, it is very rare that a vasto lorde is created. There is no information on Vasto Lorde racial abilities, although appearance wise they are very close to human in structure, and presumably, intelligence.

Abilities

Hollow

Soul-Body Separation - Two middle class Hollow, Shrieker and Acidwire, have been shown using this ability, though presumably all Hollows have the ability. This technique allows a Hollow to forcibly extract a living spirit from his/her body. It is usually not used because Hollows tend to go after Pluses, who have no physical or living body to speak of. Acidwire used this on his younger sister, Orihime, so she could see and converse with him. Shrieker used it for more sinister means, causing the young boy Yūichi Shibata to be trapped within the body of a parakeet for three months while the Hollow chases him. The soul is still attached by the Chain of Fate and can be saved, but if it is broken (in Yūichi's case) then the soul can prevent the Encroachment so long as he/she is placed inside a soulless body or animal. It seems as though Shinigami have reverse engineered this ability, as shown with the Skull Symbol. It may or may not be possible that this ability could also extract a Shinigami from their Gigai, but this would obviously prove harmful to a Hollow.

Acidic Touch - Many Hollows, like Shrieker and Acidwire, have displayed this ability, although Rukia implies that all Hollows possess this power. When a Hollow grabs a Human with their hands, the spot where the person was grabbed is shown to get burned as if acid were poured onto it. This was shown when Orihime was grabbed in the leg, and on the wound on Chad's back. These wounds seem to allow the Hollow's reiatsu to seep into the person, causing great pain to the victim. The wounds can heal if given enough time though, and it is unknown if this ability works on Shinigami or other spiritually powerful beings.

High-Speed Regeneration - The ability that allows a hollow to heal most injuries very quickly, with the exception of the destruction of the head or vital organs.

(These abilities courtesy of Bleach.wikia.org)


Menos

Gillian

Cero - By concentrating their reiatsu into a condensed form, the user is capable of releasing a blast of spiritual energy. This blast normally has a mid to high range, and has a very potent level of destruction. Arrancar are known to have personal variations of this, as well as the default option. Generally, the color for a cero is relative to the user's reiatsu color. While charging their cero, the user must remain perfectly still.

Kumon - A similar technique to garganta, Kumon opens a gateway from Hueco mundo to earth, or the other way around.

Negacion - These are fields used to rescue fellow hollows. Once activated, a yellow rectangle of light will appear around the threatened hollow, and all attacks towards it will be nullified. The hollow will then be levetated towards the creator. They are then free to travel back to hueco mundo without harm.

Adjuchas

Cero - By concentrating their reiatsu into a condensed form, the user is capable of releasing a blast of spiritual energy. This blast normally has a mid to high range, and has a very potent level of destruction. Arrancar are known to have personal variations of this, as well as the default option. Generally, the color for a cero is relative to the user's reiatsu color. While charging their cero, the user must remain perfectly still.

Garganta - This is how the arrancar, and menos-class hollow, are capable of moving through the different dimensions. 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.

Vasto Lorde

Unknown