OverviewA Hollow is a general term that describers many types of ravenous soul eating monsters.
No two Hollow are physically exactly alike, but they all of the share two obsessions. The first is hunger, because if they do not eat they die. They second is fear. Hollows are a threat to everything, including other Hollows. They have many enemies and can never feel safe. Soul Reapers can purify them by slaying them, but no Hollow wants this to happen.
A Hollow is a term that describers many types of monsters. Each Hollowed-Soul falls into one of several different classifications of power. To advance in rank a Hollow simply needs to survive. The first rank of Hollow is simply called a 'Hollow' and comprises a majority of them.
In the beginning of a Hollow's lifecycle they experience horror. Than, once they go insane, it gets much worse. When a man, or woman, or child for that matter, dies and is not ushered properly into the Soul Society they linger on the world separate from everything. They can see what's happening, but only as ghosts. Most will not go to Hell. Hell is a place for the truly egregious. Instead they find a chain has manifested on their chest. It hangs down a few feet and sprouts mouths on the final link.
That final link begins to eat itself. When that is complete the next link in the chain begins to be consumed, and so on, until the snapping maws are at the heart of the soul. When that is eaten a hole is literally carved into them that they will carry for as long as they exsist as a hollow. At this point they go mad, consumed with regrets or sadness or anger about something or someone in their life usually, and a slick substance erupts from every orifice in their head. The fluid hardens into a unique mask, their mind is all but gone, and they being looking for anything to eat. But they were relatively lucky to get this far because Hollows eat Souls, Soul Reapers, and other Hollows. Most Souls not ushered into the soul society are hunted down and made easy prey.
They can travel to a void space between the Real world and the Soul Society. Many make this trip. In the real world lingering souls are not numerous enough to feed a growing hollow, so they have to switch to hunting soul reapers who are in the real world or leave. That's usually death for the hollow in either case. In the void they call home they find a desolate waste of shadow pillars and hollowing winds. But there they can hunt each other away from the Soul Reaper's influence. Usually.
When enough of them are close enough to each other and are strong enough they begin to merge. At this point the negative energy begins to collapse into itself. What emerges is a being called a ‘Menos’, (or Minus in Spanish). It's the size of a skyscraper, shoots a huge beam known as a 'Cero' (Zero in Spanish), and is totally mindless. Unless, of course, in that fray of merging hollow one is strong enough to dominate the others. In that event it retains some fragment of what it was and continues to eat. These Menos have unique masks and typically attack other Menos with the intent of cannibalism. At this point it can evolve further if it consumes enough.