GENERAL
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.
Overview
Hollow are former Pluses (deceased Human souls) who lose their hearts to despair or regret, or they remain in the Human World for too long. Any spirit who is not guided to Soul Society by a Shinigami via Konsō may eventually turn into a Hollow.
Transformation Process
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, such as what Orihime Inoue's brother, Sora, does. 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.
Tendencies
Once a soul becomes a Hollow, it is driven by an eternal and insatiable hunger to devour other souls, living or dead, to fill the void caused by its descent. While their former obsession often becomes their first target, most Hollows will eventually become mindless creatures concerned only with finding the most potent souls to devour. Stronger and more evil Hollows are often able to retain some of their intelligence in Hollow form, making them more dangerous than the average Hollow. While most Hollows are evil, or at the very least mindless, at least a few are not, as is the case with Nel Tu and her Hollow companions. They are at least partly capable of camaraderie and loyalty, as seen with Grimmjow Jaegerjaquez and his companions while they were still Hollows; Grimmjow's subordinates voluntarily sacrificed parts of their bodies for the sake of Grimmjow's growth.
Appearance
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.
