Help improve it by adding in what you know, or look at the request below for specific details.
In the Universe of Kingdom Hearts, also called the World, there are many worlds that fall into certain realms. All of these realms are on their own plane of existence.
According to the story told by Kairi's Grandma in the Hollow Bastion's Library, long ago, there was one single world that was constantly bathed in the warmth of Light. When people began to fight over the Light within their world, Darkness appeared in their hearts, and the world was consumed. However, fragments of light remained within the hearts of children. From this light, children were able to rebuild the lost world.
However, darkness still existed, which caused the worlds to be separate from each other, divided, waiting for the true light to return. This was explained in more detail in Birth by Sleep, where it was revealed the fight was between the many Keyblade holders over Kingdom Hearts, and eventually led to the Keyblade War.
Prior to Kingdom Hearts, the worlds were surrounded by barriers, to prevent outside interference, making it impossible for different worlds to have contact. The arrival of the Heartless caused the barriers between the worlds to crumble and disappear, and some worlds were even consumed by the Heartless. Gummi Blocks are actually pieces of these broken barriers.
Those who travel between worlds are advised to avoid meddling in the affairs of other worlds, and informing their inhabitants of the existence of other worlds, in order to maintain a balance of separation. To this end, several characters, notably Sora, Donald Duck, and Goofy, magically alter their physical appearances in certain worlds to avoid standing out. For example, they become sea creatures in Atlantica, Halloween monsters in Halloween Town, jungle animals in Pride Lands, and computer A.I.s in Space Paranoids.
Travel between the worlds is normally impossible. The worlds are all protected by dimensional barriers that prevent outsiders from entering for the most part. However, these barriers can be broken, resulting in a meteor shower in that world.
In Kingdom Hearts and Kingdom Hearts II, Sora, Donald, and Goofy use the Gummi Ship to travel between worlds on the World Map, an event that serves as a mini-game between areas. Some worlds require certain special Gummi Blocks to access; for example, a special Navigation Gummi must be installed by Cid to unlock a second set of worlds in Kingdom Hearts. Once they reach them, the party can enter and then depart each world through its Save Points.
Three worlds can only be accessed by entering through other worlds and do not directly require use of a Gummi Ship
- 100 Acre Wood via Pooh's book in Traverse Town (Kingdom Hearts) and Hollow Bastion/Radiant Garden (Kingdom Hearts II).
- Timeless River via the Hall of the Cornerstone at Disney Castle.
- Space Paranoids via Ansem's computer in Hollow Bastion/Radiant Garden.
It should be noted that in Kingdom Hearts: Chain of Memories, there are no Gummi Ships; the worlds Sora visits are recreations from his memories from the first game brought to life on each floor of Castle Oblivion. This is likewise true of Kingdom Hearts Re:coded, wherein all worlds are recreations of data from Jiminy's Journal.
There are also no Gummi ships in Kingdom Hearts 3D: Dream Drop Distance, as these sleeping worlds are entered through diving, a mini-game similar in format to the original Gummi Ship mini-game but with some characteristics from the Kingdom Hearts II iteration, such as bosses that must be defeated before you are able to proceed to the world.
According to Kingdom Hearts Birth by Sleep, the pathways between the worlds are known as the Lanes Between.
There are six known ways to travel through the worlds:
- Piloting a Gummi Ship; as Gummi Ships are composed of fragments of shattered barriers, they are capable of passing through them.
- The "Star Shard" that Mickey took from Yen Sid to travel across worlds during Kingdom Hearts Birth by Sleep. The downside to this method of travel is that if the user does not know how to control the shard, the destination and time of activation will be random.
- Utilizing the Corridors of Darkness; this is a faster means of travel than the Gummi Ships, but those who use them are at great risk of being corrupted or destroyed by the darkness.
- Teleportation, used by beings that possess powerful magic; Merlin was able to teleport himself, Sora, Donald, and Goofy from Hollow Bastion/Radiant Garden to Disney Castle, as well as teleport Scrooge McDuck to Radiant Garden in Birth By Sleep. Maleficent is also capable of teleportation between worlds.
- Keyblade Glider; Terra, Aqua, and Ventus can change into their armored form and turn their Keyblades into vehicles that can travel to other worlds by using the Lanes Between.
- Time travel, such as used by Ansem, Seeker of Darkness, Young Xehanort, Yen Sid and Pete. However, Young Xehanort's method of time travel is trickier than the Yen Sid and Pete's; there needs to be a version of himself where he needs to go and he can only go backwards or forwards in time.
Stars are massive, luminous balls of plasma that are held together by their own gravity.
In the Kingdom Hearts games, each star is the representation of one specific world. Should a star stop shining, that would mean that particular world has disappeared from existence, most likely from destruction by the Heartless.
In Kingdom Hearts, many stars disappeared. In Mickey's Letter to Donald (before Destiny Islands is attacked by the Heartless), Mickey mentions the fact that the stars have been disappearing, one by one, and so he went to find out why. When Donald and Goofy first arrived at Traverse Town, they discovered a star corresponding Destiny Islands fade out of existence. They were restored after Sora and his friends stopped Ansem's plan to plunge the world into darkness.
In Kingdom Hearts II, Mufasa implies that there are stars to represent certain individuals as well, such as when he told young Simba that all the great kings of the Pride Lands are looking at them in the form of stars. Years later, Simba tries to reach out to his deceased father,and he appears to him immediately. The same implication happens in Kingdom Hearts Birth By Sleep, when Master Yen Sid detects that "Eraqus's star has blinked out", after he is struck down by Master Xehanort.
Realm of Light
The Realm of Light (光の世界 Hikari no Sekai?) is a realm made up mostly of light and makes up almost half of the Kingdom Hearts universe, similar to the Realm of Darkness. Like the Realm of Darkness, the Realm of Light has a Keyblade, the Kingdom Key, which chose Sora as its wielder.
Most of the worlds in the Kingdom Hearts universe are found here, save for those in the Realm Between, the Realm of Sleep and the Realm of Darkness. It can be presumed that most of the Worlds accessible by the Gummi Ship are either within the Realm of Light, or are related to it, like Twilight Town.
In Kingdom Hearts II, one notable citation had been made by Sora regarding the Realm of Light during the scene of Sora and Riku at the "end of the road" after defeating Xemnas: The other side... The realm of light is safe now. Kairi, the King and the others are there.
Door to Light
The door leading to the Realm of Light is called the Door to Light. It was the door Sora and Riku went through while they were stuck in the Dark Margin. It is not certain what caused the door to appear, though one reason could be, as Riku pointed out at the end of the game, the light within Sora's heart, which was brought out by Kairi's Letter (prior to reading it, Sora had willingly given up hope of returning home).
Its counterpart is the Door to Darkness.
Realm of Darkness
The Realm of Darkness (闇の世界 Yami no Sekai?) is a realm that lies opposite the Realm of Light. There are few people who have lived or are living here, such as Aqua, Ansem the Wise, and Riku, and it is instead inhabited by the Heartless and other dark beings. The only known Keyblade of the Realm of Darkness is the Kingdom Key D, wielded by Mickey. The Kingdom Hearts of Worlds' Hearts began growing in the Realm of Darkness after the Heartless plague was unleashed on the Realm of Light.
Door to Darkness
The Door to Darkness is a door in the Realm of Darkness that leads to the deepest, vastest darkness. This darkness can attract an endless swarm of Heartless. Beneath this darkness, however, is the light of Kingdom Hearts. Sora and King Mickey worked together to close the Door to Darkness in the original Kingdom Hearts. Its counterpart is the Door to Light.
The Realm Between is the third realm in the Kingdom Hearts series, and lies on the boundary between the Realms of Light and Darkness.
Few worlds exist within it, and most have some special nature or purpose. As of now, Traverse Town, Land of Departure/Castle Oblivion, Twilight Town, Mysterious Tower, and The World That Never Was are the only worlds confirmed to exist within the Realm Between. Each separate world exists closer to either the Realm of Light or the Realm of Darkness, with the only exception being Twilight Town, which exists exactly in the middle of the two realms.
The Lanes Between are a series of "roads" between the worlds in Kingdom Hearts Birth by Sleep and Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-. Ventus, Aqua, and Terra frequently use these paths to navigate the worlds. The presence of Darkness is very strong within them, forcing travellers to wear their Keyblade Armor as a safety precaution. At the end of Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-, Sora uses his Keyblade to open a portal to the Lanes Between. He then directs the Gummi Ship to travel into the portal.
To enter the Lanes, the Keyblade wielder raises his or her Keyblade to the sky, and creates a portal which appears somewhat similar to a Dark Corridor. Having opened the way, the protagonists in Kingdom Hearts Birth by Sleep summon their respective Keyblade Gliders and use them to enter. The Lanes themselves resemble the Corridors of Darkness, but with a color scheme closely resembling the Aurora Borealis.
Realm of Sleep
The Realm of Sleep is comprised of Sleeping Worlds which Sora once saved from the darkness but which did not return to the Realm of Light following Sora's victory over Ansem, Seeker of Darkness. After Sora seals the Door to Darkness with the help of Mickey, many worlds are restored from their demise, but a few are submerged into a deep slumber that prevents them from waking fully until the Keyhole in each world has been unlocked. The reasons for this slumber are unknown. Sora and Riku are sent into the Realm of Sleep by Yen Sid in Kingdom Hearts 3D: Dream Drop Distance in order to release those worlds from their slumber and for the pair to complete their Mark of Mastery exam.
The sleeping worlds found within the Realm of Sleep in Kingdom Hearts 3D: Dream Drop Distance each carry their own characteristics and unique features which set them apart from the rest. In Country of the Musketeers and Symphony of Sorcery, the dreams therein are of the past, whereas other worlds carry storylines entirely devoid of any relation to previous happenings in the Kingdom Hearts series. Traverse Town also exists within the Realm of Sleep, having been brought into existence for Joshua to have an opportunity to save Neku Sakuraba and his friends from the demise of their home world.
A World (世界 Sekai?) is a location that appears in the Kingdom Hearts series. Serving as the "levels" for the games, Sora and other characters are charged with visiting and protecting them through the course of the series. By defending the worlds from the Heartless, Nobodies, or other enemies, the games' respective protagonists keep the Darkness from spreading and the worlds from being manipulated or destroyed.
At the end of every world (in the original Kingdom Hearts) Sora, Donald, and Goofy have to fight a boss of that world (which is either a giant Heartless, or a Disney villain) which opens the Keyhole area when defeated. At the end Sora locks the Keyhole and earns a Gummi that will let him go to the next world.
Each world that appears in the Kingdom Hearts series has its own distinct look, and for the most part hold true to the movies and cartoons they are based on. Often the plot of the game in that world will be based on its original source.
Almost all worlds can be visited in the course of the game and are part of the playthrough; there are exceptions, though, since some places can be seen in cinematics but not actually accessed by Sora's party. All worlds are separate from one another, and with a few exceptions, most people are unaware of the existence of other worlds.
There are over thirty worlds that have appeared in the Kingdom Hearts series so far. Most of them are based on Disney's animated features and contain locations and characters from or inspired by the films. As a result, worlds are greatly varied in appearance, style, and nature, as their sources range from the oldest cartoons created by Disney to more current movies in the Disney canon.
The oldest source is the 1928 cartoon "Steamboat Willie", used as a basis for Timeless River, while the most recent inspiration for a world from Disney's animated features is 2014's Big Hero 6, the source for San Fransokyo. In Kingdom Hearts II, two live-action films, Pirates of the Caribbean: The Curse of the Black Pearl and Tron served as the foundation for the worlds Port Royal and Space Paranoids, respectively. In Kingdom Hearts 3D: Dream Drop Distance one direct-to-video movie, Mickey, Donald, Goofy: The Three Musketeers, which served as the source for Country of the Musketeers, makes it the first world in the series to originate from a direct-to-video film, instead of a film from the animated features canon.
In addition to the Disney-based worlds, twelve worlds are exclusives that were created by game developers Square Enix just for the Kingdom Hearts series. They are not part of the Disney film canon, nor Square Enix's games canon, but do tend to be populated with Square Enix characters, especially from the Final Fantasy series, as well as the occasional Disney character or original characters created just for Kingdom Hearts.
Some worlds have names that don't seem to accurately reflect what is found there. A famous example is Neverland in Kingdom Hearts; Sora and company get to go through Captain Hook's ship and fly around the Clock Tower, but never actually reach the island Neverland where Peter Pan and Tinker Bell live. To further confuse things, some worlds in Kingdom Hearts II contain what could almost be considered sub-levels or second worlds within them, such as the Underworld at Olympus Coliseum or Christmas Town in Halloween Town.
World party members
Kingdom Hearts Birth by Sleep does not feature set party members and only allows a solo party for most of the game, except for a handful of battles, in which one or two additional members are assigned to your party. Instead, the game takes advantage of the Dimension Link (D-Link) system, a descendant of the Summon system of previous games.
In certain worlds, a character from that area will be available to fight, and can replace Donald or Goofy in the party. Kingdom Hearts had six such worlds with party members.
Sora received cards in Castle Oblivion. He is sent to worlds from his own memories. The past Disney party members from Kingdom Hearts have become memory cards.
Kingdom Hearts 358/2 Days has several worlds that Saïx sends Roxas to on missions, often along with other members of the Organization.They are also available to play as in mission mode
Kingdom Hearts II
Kingdom Hearts II increased the number of worlds with party characters to nine, putting a party member in almost every Disney-based world. Three characters from Kingdom Hearts returned for battle: Aladdin, Jack Skellington, and Beast, although Beast was moved from Hollow Bastion to his own world. Ariel reappeared in Atlantica, but no longer as a party character, Peter Pan became a Summon, and Simba, formerly a Summon, became a party member in his homeworld of the Pride Lands.
The game added two party members with the same first name as another; Jack Skellington and Jack Sparrow; Sora differentiates between the two by referring to Jack Sparrow as Captain. To make the world-based party members more powerful, special attacks were developed for each character that Sora could call on in the heat of battle. Kingdom Hearts II also included the first Final Fantasy party character, Auron, and allowed Sora to fight alongside his best friend, Riku.
Kingdom Hearts coded and Kingdom Hearts Re:coded
The worlds in Kingdom Hearts coded and Kingdom Hearts Re:coded are the same as the first game, but the party members are different.
Kingdom Hearts 3D: Dream Drop Distance
In the worlds of Kingdom Hearts 3D: Dream Drop Distance, the party member set is replaced with a collection of Spirit Dream Eaters, which can be created by recipes collected throughout the game, as well as by combining Dream Pieces together.