Bleach is a supernatural "monster hunter" anime in a similar vein as Demon Slayer, Chainsaw Man, and Jujutsu Kaisen. Anime series like these are all about brutal monsters and the trained professionals who hunt them to protect humanity. While Jujutsu Kaisen has curses and Demon Slayer is all about Muzan's demon minions, Bleach focuses on monstrous spirits called Hollows, which are a major factor in the first story arc in particular.
Right away, Bleach establishes a few basic facts about Hollows, such as the fact that they're born from the souls of the restless dead. All Hollows have skull-like masks and a circular hole somewhere in their bodies, hence their names, and Hollows will eat the souls of the living and dead alike. That's all protagonist Ichigo Kurosaki needs to know when hunting these malevolent spirits, but Bleach fans would like to know more about exactly how Hollows operate and the themes they represent.
10 Some Hollows Have Bounties
An oft-overlooked and minor part of Bleach lore is the bounties that the Soul Reapers put on Hollows. In the Substitute Soul Reaper story arc, the genius scientist Kisuke Urahara reviewed a few notable Hollows that Ichigo had slain recently, stating that most of them had negligible or no bounties to collect. However, the vicious Hollow named Shrieker was worth 5,000 Kan.
Kan is the currency of Soul Reapers, and if some Hollows are worth cash payments, then they are evidently the most dangerous non-Menos Hollows out there. This obscure bit of Bleach lore opens the possibility of bounty hunter Soul Reapers who patrol the world of the living to get rich, though the story never went in that direction.
9 Hollows Aren't Judged For Their Crimes as Spirits
Hollows are dangerous spiritual entities that Soul Reapers are always ready to fight, but Soul Reapers don't generally consider Hollows evil. Many Hollows are warped souls with animalistic intelligence that may not be entirely aware of what they are doing. Also, when a Hollow is slain, it's not just entirely destroyed — it's forgiven. A defeated Hollow is purified, and then sent to the Soul Society.
Thus, Hollows are not condemned as evil creatures, setting them apart from Muzan's demons and the devils found in Chainsaw Man. Instead, a Hollow is only sent to Hell for eternal punishment if the soul committed horrible crimes as a mortal human, as was the case for the Hollow called Shrieker, who was the Hollow of a murderer.
8 Hollows Can Visit the Soul Society
In general, the Soul Society is a peaceful place that's free of Hollows and monsters, as fans saw in the fondly-remembered Soul Society story arc. The citizens there don't have to worry about Hollow attacks in their daily lives; instead, most Hollows are either in the world of the living or in Hueco Mundo.
However, there are exceptions, as fans saw in the Turn Back the Pendulum story arc in Bleach. Seemingly out of nowhere, some Hollows arrived in the Soul Society, and it fell to Kensei Muguruma, Squad 9's Captain at the time, to slay them with his shikai, Tsuchikaze. Such an occurrence is evidently rare, but even so, Kensei was only mildly surprised at most; this suggests that while such attacks aren't common, Captains are always on alert for them.
7 Some Hollows Can Sustain Themselves on Ambient Spirit Energy
Most Hollows sustain themselves on other spirits, and they tend to eat spirits to try and fill the emptiness in them. Some Hollows even eat their own kind in a bid to get stronger, such as Menos Grandes who are trying to evolve. However, as Ichigo saw in the bleak world of Hueco Mundo, some Hollows don't have to go that far.
Ichigo spotted a small, lizard-like Hollow that simply burrowed under the white sands to escape Ichigo's team, unwilling to even try and fight them. It was noted that perhaps the smallest Hollows, with equally small appetites, can sustain themselves on the ambient spirit energy of worlds like Hueco Mundo without having to eat any souls at all.
6 Unmasked Hollows Tend to Have Double Letters in Their Names
A variety of Arrancars, or unmasked Hollows, have a double-letter pattern in their names. The ten Espadas follow that theme, such as Ichigo's shonen-style rival Grimmjow Jaegerjaques, along with the bizarre Aaronerio Arruruerie, Coyote Starrk, Tier Harribel, and others. Some of the lesser Arrancars follow this pattern too, such as Dondochakka, Gantenbainne Mosqueda, Apacci, and Menoly Mallia.
Even the words "Hollow" and "Arrancar" have double letters in them to reinforce this theme and remind Bleach fans that even if Arrancars have humanoid appearances and zanpakuto, they are still Hollows, complete with their holes. Curiously, some lesser Arrancars buck this trend, such as Di Roy Rinker, Edrad Liones, and Mila Rose.
5 Menos Grandes Stop Their Evolution When Injured
Ordinary Hollows don't give much thought to evolution and transformation, but Menos Grande do. Menos are stronger Hollows whose bodies are made up of all the Hollows they consume, and many of them have human-level intelligence. Menos Grande also come in three classes, with Gillians being the weakest, Adjuchas being the middle class, and Vasto Lorde being the strongest.
Menos can eat other Hollows to get stronger and evolve, and if they go hungry for too long, they will regress into hive-minded Gillains, a fate that they fear. As Grimmjow's Hollow team showed in a flashback, however, Menos will stop changing if they are wounded and have a chunk of flesh bitten off. Doing so means the Menos can never evolve, but it also means they won't regress. Shawlong and his fellows thought that was a fair trade, so they asked Grimmjow to bite their flesh and freeze their evolution forever.
4 Hollows Represent Despair and Empty Strength
The monsters in "monster hunter" anime series often represent certain themes; they aren't just excuses for the heroes to gear up and fight some critters. Tokyo Ghoul's ghouls represent the moral ambiguity of humans and ghouls attacking each other, for example, and the demons in Demon Slayer are actually victims deep down, which is why Tanjiro Kamado cries for them.
The Hollows in Bleach represent the emptiness of despair and meaningless power. Hollows are named after the round holes in their body, and nothing can ever fill that agonizing void, not even after eating hundreds of souls. Hollows have no salvation, even if they get stronger and rule Hueco Mundo, like Baraggan once did. Strength alone means nothing if a being is doomed to emptiness, so it's actually in a Hollow's best interest to be slain, even if they can't stand the thought of it.
3 Hollows Don't Show Bleach Fans How They Become Vasto Lordes
Ordinary Hollows can become a Menos Grande by cannibalizing their own kind and fusing with them to become a powerful amalgamation. Then, a Gillian-class Hollow can keep eating other Hollows to power up and become an Adjuchas, such as Grimmjow. However, it's not clear how an Adjuchas can evolve again to become a Vasto Lorde.
Some Hollows thought they could just keep cannibalizing other Hollows in Hueco Mundo to become Vasto Lordes, but anecdotal evidence suggests that's not how it works. It's been theorized that certain Hollows have some innate trait that allows them to evolve into Vasto Lordes, which isn't something that a Hollow can obtain. Luckily, Vasto Lordes are so rare, that Soul Reapers don't have to put much thought into this and focus on hunting regular Hollows and lower-class Menos instead.
2 A Hollow's Mask and Powers Represent Its Human Heart
Hollows, unlike Chainsaw Man devils, were once human and aren't creatures from Hell or some other realm. Thus, a Hollow's mask and powers represent the human being it once was, meaning its traits aren't entirely random. Bleach lore states that when a soul becomes a Hollow, that spirit's human heart is mutated to form the Hollow's skull mask and powers.
A good example is Shrieker, whose abilities are ideal for a hunter. Shrieker, as a human, was a murderer who was a twisted hunter, and that continued in the afterlife. This makes Hollows roughly similar to the demons in Demon Slayer, who sometimes have powers or visual themes to represent the humans they once were, such as Daki wishing to be beautiful and in control of her own life.
1 Hollows Are Venomous to Quincies
Hollows are dangerous to mortal humans, Pluses, and Soul Reapers, but they are downright poisonous to the Quincy tribe. Hollows can use their physical attacks against Soul Reapers to inflict injuries, while a Hollow's very essence is a lethal venom to Quincies. A Soul Reaper can gain Hollow powers to become a Visored, like Shinji and Hachigen, but a Quincy can't do that.
Since Hollows are so dangerous to Quincies, the Quincy tribe makes a point of destroying Hollows completely with their spiritual attacks, not bothering to purify Hollows and return their souls to the great cycle of spirits. Unfortunately, doing that disrupts the balance of souls between the mortal world and the Soul Society, which is why the Soul Reapers eventually went to war with the Quincy tribe. It's also why Masaki Kurosaki needed Isshin Shiba the Soul Reaper to save her when White's essence started poisoning her from the inside.
