Hollow (虚 (ホロウ), horō) are a race of creatures which are born from Human souls who, for various reasons, do not cross over to Soul Society after their death and stay in the Human World for too long. They are corrupt spirits with supernatural powers which devour the souls of both living and deceased Humans. Hollows settle in Hueco Mundo, but can cross over to the Human World and Soul Society. Hollows are the opposite of Humans.
Hollow are former Pluses (deceased Human souls) who lose their hearts to despair or regret, or they remain in the real world for too long. Any spirit who is not guided to Soul Society by a Shinigami via Konsō may eventually turn into a Hollow.
The majority of ghosts peacefully move on to the Soul Society after their deaths. However, some stay in the living world to fulfill certain goals, such as looking after a loved one, protecting a particular location, or pursuing a grudge from beyond the grave. However, with only limited abilities to interact with the living, and by definition tied to their goals, many ghosts find they are unable to cope when their loved ones move on or old enemies forget them. At this point, some will become dangerously obsessed with fulfilling those goals, even if the means to fulfill them renders the soul empty: for example, killing anyone who comes near a loved one in order to keep them from hurting that person. In doing so, the ghost warps into a Hollow and, ironically, often makes the focus of their obsession into their first victim. This can be prevented if a Shinigami gives the soul a Soul Burial before it turns into a Hollow.
The process by which a soul becomes a Hollow usually takes months to years to complete, reflected by the length of the soul's Chain of Fate, which usually resides in the center of their chest, and connects to their bodies, if present. The Chain of Fate slowly corrodes over time, coinciding with the degradation of the soul's ties to the living world. The process can be accelerated under certain conditions, such as being attacked by a Hollow, giving into extreme despair, or using certain spiritual powers. When the chain completely decays, they become a Hollow when a hole opens up on their chest where the chain was attached, signifying they have lost their heart.
All Hollows wear masks, which are formed from the hearts they had lost as Humans. This mask protects the naked instinct which is left in a being after the loss of its heart, and obscures the original identity of the Hollow. The masks of different Hollows can differ greatly in shape and form, but are always white and skull-like. The mask can be shattered, temporarily allowing the Hollow to regain its original identity, but it will regenerate after a short period once the Hollow takes over again.
Hollows have widely varying physical characteristics, rarely appearing the same. In terms of appearance, Hollows can range in appearance from furry (like mammals) to lizard-like to outright demonic. Hollows vary in size, though it tends to be fairly consistent among the various classifications of Menos. In Hueco Mundo, some Hollows are no larger than common pets. Most common Hollows are about twice the size of an adult Human, though it can vary to a degree.
Like Shinigami, Hollows have a multitude of combat skills and abilities, most of which are usually unique to specific Hollows. The more powerful Hollows, however, have access to a range similar techniques of greater power or use than their own unique skills. Hollows usually acquire energy to power their abilities by devouring souls, but smaller ones can ingest energy from the atmosphere when in realms with a high concentration of spiritual energy, such as Hueco Mundo.
- Soul-Body Separation: Two middle class Hollow, Shrieker and Acidwire, have been shown using this 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. The soul is still attached by the Chain of Fate and can be saved. But if it is broken, the soul can prevent the Encroachment so long as he/she is placed inside a soulless body or animal.
- Acidic Touch: Many Hollows, like Shrieker and Acidwire, have displayed this ability, although Rukia implies all Hollows possess this power. When a Hollow grabs a Human with their hands, the spot where the person was grabbed is burned as if acid were poured onto it. 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, and it is unknown if this ability works on Shinigami or other spiritually powerful beings.
- Cero (虚閃 (セロ), sero; Spanish for "Zero", Japanese for "Hollow Flash", 'Doom Blast' in the Viz translation) is a highly powerful and relatively swift energy blast which can be fired from various body parts, such as the mouth, tongue, hand, or fingers. Only Menos, Arrancar, and Visored have used Cero attacks, and it appears stronger entities can use it more efficiently and forcefully. With the exception of a few Cero techniques and users, Cero requires some time to be used effectively, so the enemy must be not moving, busy with something else, or too badly wounded to even simply dodge it. There exist a number of variations of Cero, which are usually unique to the Arrancar using it.
- Garganta (黒腔 (ガルガンタ), garuganta; Spanish for "Throat", Japanese for "Black Cavity") is how Hollows and Arrancar move to and from Hueco Mundo.
- Negación (反膜 (ネガシオン), negashion; Spanish for "Negation", Japanese for "Counter-Membrane") fields are used to rescue fellow Hollows. Only the Gillian-class Menos have used it. Espada have a special cube-shaped device called Caja Negación (反膜の匪 (カハ・ネガシオン), kaha negashion; Spanish for "Negation Box", Japanese for "Anti-Membrane of Negation") (Caja de Negación in the English dub), meant to simulate Negación fields, which are used to trap and punish their subordinates if necessary.
- High-Speed Regeneration (超速再生, Chōsoku Saisei; lit. "Ultra-Fast Regeneration") is an ability which allows a Hollow to heal most injuries very quickly, with the exception of the destruction of the head or vital organs. Most Arrancar gave up this ability for far greater strength.
- Soul-body Separation: all Hollows can do this
- Acidic Touch: all Hollows have this
- Cero: at least 7 in SEI; only Gillian-class Hollows can use Ceros
- Garganta: at least 3 in SEI
- Negacion: at least 6 in SEI; only Gillian-class Hollows can use this
- High-speed Regeneration: at least 13 in REI; may not be used in battle