The games are influenced by their parent franchise, Final Fantasy, [105] and carry its gameplay elements over into their own action-based, hack-and-slash system. Nomura did this because he feels that games should have room for fans to speculate and use their imagination.

As they read the letter, they learn of Xehanort's true identity as a Keyblade Master who sought the secrets of the Keyblade War which created their current reality and the fates that befall the apprentices of his friend Eraqus during the events of Kingdom Hearts Birth by Sleep: Terra having ended up possessed by Xehanort and made into the original being that Xehanort's Heartless and Xemnas were split from; Ventus sacrificing his heart by fighting his dark counterpart Vanitas, while trying to prevent Xehanort from recreating the legendary χ-blade which allows the user to control Kingdom Hearts, and hiding within four-year-old Sora's heart; and new Keyblade Master Aqua trapped within the Realm of Darkness while an amnesic Xehanort ended up Ansem's apprentice alongside his accomplice Braig (Xigbar's original self).

The main theme songs for the Kingdom Hearts games were written and performed by Japanese pop star, Hikaru Utada.

DiZ, revealed to be the real Ansem, attempts dissipating some of the artificial Kingdom Hearts before being engulfed in an explosion when his extraction device self-destructs and is sent to the Realm of Darkness. Sora, Donald, and Goofy fight their way through Castle Oblivion and eventually encounter and destroy Marluxia, the lord of the castle, thus freeing Naminé from his grip. At the same time Sora ventured through Castle Oblivion, Riku ended up in the basement levels and ascends to the ground floor with Mickey's aid while dealing with a Replica of himself that fought Sora.In the month after its release in North America, Kingdom Hearts II sold an estimated 1 million copies.

The two main themes are Hikari, from Kingdom Hearts, Chain of Memories, Birth by Sleep and re:coded" and Passion, from Kingdom Hearts II and 385/2 Days.In Kingdom Hearts, Chain of Memories, Kingdom Hearts II, and Kingdom Hearts III, the journal is kept by Jiminy Cricket, who was appointed by Queen Minnie as the royal chronicler. Unlike Kingdom Hearts for the V CAST and Kingdom Hearts coded, this game does not have a storyline and focuses more on socializing.