While the player character in Dark Souls can be in a "hollow" state, the player never truly does go hollow. At least, not in the sense that NPCs and enemies have gone hollow. According to Dark Souls' mythology, the undead are condemned to repeatedly wander Lordran in search of a cure, being unable to permanently die. But for virtually all such undead, this quest is futile. An undead can temporarily stave off hallowing by absorbing souls or infusing themselves with the humanity of someone else. Eventually, an undead dies one too many times, or is worn down by the daily grind of collecting souls, and loses the will to go on. When this happens, that undead becomes hollow, loses his sanity and free will, and continues to wander the world as a mindless zombie attacking any un-hollowed that it encounters on sight.
It is unknown how many "Chosen Undead" are brought to Lordran, but the Crestfallen Warrior at Firelink tells us that many have come before you. Is it possible that all hollows in Lordran were at some point "Chosen Undead", tasked by Frampt to retrieve the Lordvessel and re-kindle the dying flame? Probably not. A great deal of the hollows that you encounter in the game were likely former residents of Lordran, and there was no need to select a "Chosen Undead" until Gwyn's power faded to a "cinder", and the fire began to die. This presumably took a very long time - a whole "age", to be precise.
The Crestfallen Warrior informs us that we are not the first "chosen undead",
and suspects that we won't be the last either.
Avoiding hollowness with purpose
Many undead adventurers wandered into Lordran (or were abducted and taken there), and they struggle to hold onto their precious humanity for as long as possible, fighting for their lives in the fear that they, too will go hollow. Some, like the Crestfallen Warrior, resign themselves to the inevitability of hollowness, and find a sense of purpose in warning other new arrivals that they, too, are doomed. Others pursue some seemingly impossible goal or objective in the hopes that the journey will provide them with the sense of purpose necessary to avoid (or at least delay) hollowing. And yet others have taken up crafts or vocations such as blacksmithing, vending, or guarding something in order to keep them focused and avoid hollowing. Keeping such a goal may help keep an undead partially lucid, but they also seem to begin to forget everything else, and only the knowledge of their quest or craft remains.
Going on "one final quest" seems to provide adventurers with enough focus to hold back hollowing.
But hollowing isn't just a thematic element reserved for non-player characters; hollowing is also a mechanic in the game that affects the player. Whenever the player character dies, you are reborn at the last bonfire in a hollowed state, unable to summon help from allies until you restore your humanity through the consumption of someone else's humanity. In Dark Souls II, hollowing further handicaps the player by cummulatively reducing your total health each time you die, and only restoring your humanity can refill your health meter. In both these cases, the player is not truly hollow; you are only in a state of partial hollowing.
It's unclear whether non-player characters are able to die and restore their humanity, or if deaths contribute to an irreversible progression towards hollowness. There are, after all, apparently hollowed NPCs such as the undead merchant in the Undead Burg and blacksmith Lenigrast in Majula who are sane enough to have kept their shops open. The presence of NPC summon signs hints at the possibility that they, too, are capable of restoring their own humanity through the same mechanisms that you can, but the game itself justifies this with ambiguous appeals to "time distortion" and parallel realities that obfuscates the matter - particularly where Solaire and Lautrec are concerned.
Summoned NPCs may recover humanity as you do, or they're from another time or dimension, or both.
Solaire's dialogue refers to "heroes centuries old phasing in and out.". Solaire may be using the words "world" and "time" interchangeably. This seems to be the game's justification for how summoning works: you may be literally summoning someone from a bygone era into your own time period. Anytime, you are summoned to someone else's world, you are also being transported to another time (past or future, depending on whether or not you finish the game). Solaire and Lautrec seem to somehow come from another time or dimension, but other characters definitely seem to exist within your world and time: Andre, the Crestfallen Warrior, Rhea and her companions, Big Hat Logan and his apprentice, and so on are all undead who have seen many other "Chosen Undead" come to Lordran seeking their destiny.
In any case, it's not until an undead "gives up" that the hollowing process becomes complete. What do we mean by "giving up"? For an NPC, it means that they gave up on life and went hollow, and the player typically ends up putting them down. For the player, it means that you stop playing the game. As long as you continue to play the game, then your character will continue to hold onto a sliver of humanity and maintain his or her sanity for a little while longer. When you put down your controller for the last time, you have condemned your character avatar to finally succumbing to hollowness, whether you recognize it or not.
Hollowed-looking vendors still hold onto a sense of purpose that maintains some amount of free will and sanity.
This happens to many NPCs in the first Dark Souls games. The Crestfallen Warrior, Rhea of Thorolund, Seigmeyer, Logan, Lucatiel, and other NPCs all become hollow over the course of the game. This happens when they either give up on their objectives, fulfill their objective (and thus have no further purpose in life), or outright go mad. Even Gwyn appears to have become hollow when you fight him in the Kiln of the First Flame, as the flame has faded, and he is no longer able to restore it. The version of Gwyn that you fight is an irrational husk of a god that attacks the player on-sight, regardless of whether you are there to take Gwyn's place and rekindle the flame, or if you are there to kill him and usher in the Age of Dark.
One notable character who does not go hollow is Solaire in the first Dark Souls game. His story has two outcomes. He either finds the Sunlight Maggot and goes insane, but does not go hollow, or he survives long enough to be an optional summon for the final boss fight with Gwyn. In both cases, Solaire avoids going hollow despite concluding his character arc by either finding his own sun, or by realizing that his sun doesn't exist. This fuels the speculation that Solaire may be the forgotten God of War whose mistake resulted in exile from Anor Londo and being expunged from the annals.
Many NPCs in Dark Souls go hollow when they lose their will or reason to live.
But what would happen if the player gave up on his or her purpose and went hollow?
Thinking of giving up on the game as "causing your character to hollow" isn't something that is actually a part of the game. It's only something that is implied, and even then, only to those of us who liked the game enough (and had the patience) to play it long enough to understand what hollowing and humanity mean in the context of the game's world and the player character. So all those players who get stomped by the Taurus Demon (or Last Giant in Dark Souls II) and then never play the game again simply aren't aware of the profoundity of their decision within the game's own lore. They aren't aware that their act of "giving up on the game" is, in some meta-sense, actually a way of completing the game. They ended their character's narrative arc in a perfectly legitimate and viable, canonical way. They just didn't see a credits screen for doing so.
In one sense, Dark Souls hits an idealized realization of an emergent narrative. Every action that the player takes - including not playing the game - is a progression of the narrative and a character arc in some way.
Player hollowness as a hypothetical mechanic
Those players' decision to complete their arc by "giving up" and going hollow doesn't affect anyone else who is playing the game in the way that continuing playing the game influences everyone else - whether you use the multiplayer mechanics or not. At best, a quitter's bloodstain will warn some people of how not to die, but that's only temporary. Those bloodstains will disappear eventually, leaving no trace that player or character profile ever existed. But they could do more. A lot more!
1.) Fully human character.
2.) Partially hollowed character looks like a zombie.
3.) Fully hollowed character looks like a very decayed zombie.
Player characters in Dark Souls II become increasingly hollow as they die repeatedly,
but they never actually complete the process of turning hollow.
What if the player character hollowing were an actual game mechanic? Dark Souls II took a small step in this direction by making repeat deaths incrementally decrease your max HP while also making your character avatar look more decayed. But you still never truly "go hollow".
This mechanic from Dark Souls II could be expanded to include a perma-death feature. After you die too many times and hit that cap of HP-reduction, one more death results in the full hollowing of your character, a permanent "Game Over", the deletion of your character profile, and maybe even an end credits screen and an achievement / trophy for truly losing the game.
Supposedly, the original Demon's Souls might have been scoped to have a "true death" feature. It's unclear whether this was a separate "hardcore" mode, or if it was a standard feature of the core game, or if it was a New Game+ mechanic. This doesn't seem like it was intended to be a perma-death though, but rather just a variation on what the actual death mechanics ended up being. If you died in human form, you became a ghost. If you died as a ghost before reaching your bloodstain, you lost all accumulated souls. In this iteration of the game's design, reaching your bloodstain also restored your human form. It's possible that dying as a ghost might have had other more severe effects, such as deleveling your character and/or equipment. This was apparently changed so that regaining your human body could only be accomplished by defeating bosses, and the "true death" feature was scrapped.
NPCs in Demon's Souls could be killed as phantoms, and then never re-appear in the Nexus.
Does this signify an in-universe perma-death? Or is it just a gameplay contrivance?
Dying in soul form in Demon's Souls, or dying as a hollow in Dark Souls (or at the last stage of hollowness in Dark Souls II), would thematically work as a perma-death. After all, the player will kill the ghost or hollowed form of most (if not all) of the NPCs that you meet in those games. But there's good reasons why perma-death features are generally reserved for short, roguish-style games such as F.T.L.: if you're going to have to invest 20, 50, or 100 hours (or more) into playing a game and developing a character, then you probably don't want to have all that time and progress erased by an unlucky circumstance or single screw-up. It works fine in games that can be completed in an hour or two, but not for games that take weeks or months to complete. That doesn't necessarily mean that perma-death couldn't be an option for a Souls-style game, but it should certainly not be the default. I could definitely see room for a Dark Souls game to have a "hardcore" mode that enables the possibility of perma-death for players who are willing to take the risk.
Player Character Hollows
There is another way that true hollowness could have worked in Dark Souls. After a player character "goes hollow", the game server could generate an A.I.-controlled avatar of that player's character on the servers that can be spawned into other players' games as an enemy around the location of that player's furthest progress into the game. In effect, a player's character becomes a mindless zombie, indiscriminately attacking other player characters that encroach upon its domain. Defeating that enemy could provide a decent reward, such as a rare item, upgrade stone, humanity, or maybe even a piece of equipment from that player's inventory.
Such a mechanic would be similar to the "pawn" system from Dragon's Dogma. There's already precedents for such mechanics in the first Dark Souls. The Gravelord Servant Covenant allows players to "curse" other players' worlds with extra enemies. Another mechanic is Vagrants (a.k.a. "Drift Items"), which can be spawned randomly in players' worlds if another online player drops a valuable item on the ground in their world, or dies and loses a very large sum of souls or humanity. They come in two flavors: good and evil. The good ones will flee from the player and disappear similar to crystal lizards (but do not respawn if the level is reloaded), and the evil ones will attack the player. If killed, they drop rare items. They are extremely uncommon, and I've only seen (I think) two of them in the entire time that I've played Dark Souls.
The mechanical precedents above could be applied to player hollowness in Dark Souls. Any player who suffers a perma-death in the game's "hardcore" mode would have their character's profile information saved onto the servers. That character could then spawn in other players' (or characters') games as a hollow NPC enemy around the location where they perma-died.
A softer version of this could also be applied if perma-death is not used. If a player dies while hollow, the game could similarly save a copy of their character profile on the server. That avatar could then spawn in other players' games, but the owner could still continue to progress that character normally. If they die again in hollow form, then the copy of that profile on the server could be updated with the character's new stats, equipment, and the location of their death.
Dropping a valuable item on the ground, or losing a large sum of humanity or souls,
can result in Vagrants (a.k.a. "Drift Items") spawning in other player's worlds.
Your own hollowed characters could maybe even show up in games using other character profiles that you've created on the same system or PSN, Live, or Steam account! Though, the rewards for defeating such characters should probably be minimized in order to avoid some player exploitation and item-duplication possibilities. This would provide you with a potentially fun, challenging, and maybe even thought-provoking, callback to previous attempts at playthroughs. It's the same basic principle as the NPC invasions that already occur in the Dark Souls games, except that this model is a more user-driven one.
Making an achievement based on turning your own character hollow would even provide an incentive to completionists and achievement-seekers to use the feature; thus, providing a guaranteed pool of player-character-generated hollows for others to fight. Perhaps there can even be a rating or indictment system for avatar hollows that would increase or decrease the odds of that avatar being stored on the servers and continuing to show up in other players' games (just like with messages). That way the community can hopefully retain particularly good or interesting player hollows, while weeding out unfair or repetitive ones.
Going hollow by giving up
A more abstract option could be for the game to adopt the idea of the player "giving up" as causing their character's hollowing. Each time you exit the game, your character's avatar would be uploaded to the server (similarly to the above proposal for perma-death). After some period of inactivity (maybe a few months?), that character is considered to have given up on his or her quest and goes hollow. That character avatar would then be available to spawn in other players' games.
Scholar of the First Sin populated the world with scripted, minor NPC phantoms.
What if these could be replaced with hollows based on other players' characters, telling their stories?
If the player loads back up that profile, then their character could be deactivated on the server, since that character no longer has the "given up" status. If the character becomes inactive again, then it can become available to act as an NPC hollow enemy with the most recent stats and equipment.
Would player hollowness be a welcome addition?
So in summary, I'm proposing a system in which player characters can "go hollow" just like their NPC counterparts. This mechanic can be tied to an optional "hardcore" mode that has perma-death. Suffering a perma-death would result in an A.I.-controlled avatar of your character being able to appear in other online players' games, or possibly in your own game with a different character profile. Alternatively, "going hollow" could be tied to some arbitrary time limit of inactivity on a specific character's profile in order to emulate the idea of the character "giving up" on his or her quest. Such a system could create an evolving, organic game world with player-driven hollows showing up in non-scripted locations, telling their own little stories of other players who struggled valiantly, but failed to complete the game, and have since gone hollow.
Would such a system be a welcome part of your Dark Souls experience? Maybe we'll see something like it in Dark Souls III.