Each Bleach human character consists of a body animated by a spirit identical in appearance. In the unusual event that a spirit is separated from his or her living body, it is possible to see a connective structure known as the chain of fate (因果の鎖, inga no kusari), which attaches the spirit to its body and allows it to return to it.When a human dies or the chain of fate is broken by force, the spirit cannot come back and becomes known as a plus (whole in the English translation). If the plus has many regrets, or strong emotional connections to a location, object, person, etc. he/she becomes attached to that location, object, person, etc. by the chain of fate. After some time, his/her emotions will eventually corrupt and gain control over the soul, which transforms into a hollow. Additionally, a broken chain of fate can eventually corrode completely, which also leads to the transformation into a hollow. It is also possible for a spirit to use his/her abilities as a Soul Reaper to avoid turning into a hollow, instead becoming a visored.
Once a plus becomes a hollow, it gains a monster-like appearance, a skull-like mask, a large hole somewhere through its body, and an animalistic level of intelligence. Hollows crave other human souls, living or dead, to eat in an attempt to fill the void left in their hearts, starting with the souls of the people that they had loved the most in their past lives.$ Additionally, some hollows consume other hollows, eventually evolving into menos. The hollows reside in the desert world of Hueco Mundo when not scouring the human world for souls to consume.
As hollows evolve, they gain access to unique skills and abilities, fueled by the souls they have consumed. The most common of these abilities is cero (虚閃（セロ）, sero, Japanese for "hollow flash," Spanish for "zero," called "doom blast" in the Viz manga), a high-powered energy blast that can be fired from various body parts, such as the mouth or hand. If a hollow has its mask removed, it becomes an arrancar, and gains additional abilities and a more humanoid appearance.
There are two main duties of the Soul Reapers when dealing with hollows: to perform a ritual known as a soul burial (魂葬 (こんそう), konsō) on pluses before they become hollows, and to slay hollows to purify their souls of all sins committed while in that form, so that they become pluses once again. When either of these duties are fulfilled, the plus goes to Soul Society.