The central theme of Kingdom Hearts is the power of the heart and the bonds of friendship. The protagonist, Sora, derives his strength not from physical prowess but from his connections with others. Phrases like 'My friends are my power!' underscore the idea that light and hope can overcome even the deepest darkness when people stand together.

A Keyblade is a mysterious, key-shaped weapon that can unlock hearts, worlds, and pathways. It chooses its wielder based on the strength of their heart. Keyblades are effective against Heartless and Nobodies, and can be used to seal the 'Keyholes' of worlds to protect them from darkness. There are also different types, such as Keyblades of Light, Darkness, and Heart.

The Heartless are physical manifestations of the darkness within people's hearts. When a strong-willed person loses their heart to darkness, their body and soul remain as a 'Nobody' (e.g., Organization XIII). The Unversed, introduced in 'Birth by Sleep,' are nascent emotions spawned from the negativity of Vanitas. Together, they form the primary enemy factions in the series.

The 'Dark Seeker Saga' (or Xehanort Saga) is the first major story arc of the Kingdom Hearts franchise. It spans from the original game through 'Kingdom Hearts III.' It revolves around the conflict between the Guardians of Light (Sora and friends) and Master Xehanort, who seeks to recreate the legendary ﾏ・blade (Chi-blade) and start a new Keyblade War to balance light and darkness.

Disney worlds serve as the setting for much of Sora's journey. Each world (e.g., Agrabah, Olympus, Arendelle) retains its own storyline, often mirroring the movie plot but integrating Sora, Donald, and Goofy. The villains of these worlds often ally with the main antagonists (like Maleficent), while the heroes help Sora understand different aspects of the heart.

Master Xehanort is the main antagonist of the Dark Seeker Saga. He is a Keyblade Master who believes the current world is too unbalanced towards light and seeks to forge the ﾏ・blade to unlock Kingdom Hearts. His goal is to purge the world and create a new one where light and darkness exist in perfect equilibrium, even if it means destroying the current universe.

Sora is the main protagonist. Roxas is Sora's Nobody, created when Sora briefly became a Heartless in the first game. Ventus is a Keyblade wielder from the past whose fractured heart took refuge inside a young Sora's heart to heal. Because Ventus's heart was inside Sora when Roxas was created, Roxas took on Ventus's appearance rather than Sora's.

Organization XIII is a group of powerful Nobodies led by Xemnas (Xehanort's Nobody). They seek to complete Kingdom Hearts to regain their own hearts and become whole again. Members include Axel, Xion, Saﾃｯx, and others, each controlling a specific element and type of lesser Nobody. In 'Kingdom Hearts III,' a 'Real Organization XIII' is formed to serve as the 13 Darknesses.

King Mickey Mouse is a Keyblade Master and the ruler of Disney Castle. He is a mentor figure to Sora, Riku, and Kairi. He operates largely behind the scenes in the early games, investigating the threat of darkness, but becomes a frontline fighter and key Guardian of Light in later installments, wielding the Kingdom Key D (from the Realm of Darkness).

Yes, absolutely. Games often labeled as 'spin-offs' (like 'Chain of Memories,' 'Birth by Sleep,' and 'Dream Drop Distance') contain critical plot points, character backstories, and lore that are essential for understanding the main numbered titles, especially 'Kingdom Hearts III.' The series is one continuous narrative spread across multiple platforms.