The number thirteen is a recurring number throughout the series:
*It is primarily associated with Organization XIII (and by extension, Mushroom XIII, Finest Fantasy 13, the thirteen floors of Castle Oblivion).
*Riku's Limit "Session" includes an ability named "XIII Blades".
*There are thirteen letters in "Kingdom Hearts".
*The ability that Sora uses in the first Kingdom Hearts, "Ars Arcanum", consists of thirteen attacks if you select the follow-up attack.
*In Kingdom Hearts Birth by Sleep, not counting the additional matches in the Final Mix version, the Mirage Arena has thirteen matches.
*The Ultima Weapon in Kingdom Hearts II requires 13 Orichalcum+ for item synthesis.
*The sum of all the letters in the names Sora, Riku, and Kairi is thirteen. The same is true for Roxas, Sora, and Xion, as well as Roxas, Axel and Xion, and Ventus and Vanitas. Also, if one does not count the dash, there are thirteen letters in Terra-Xehanort.
*In the opening movie of Kingdom Hearts the letter "M" in the word "Kingdom" surges towards the screen distinctly, with "M" being the 13th letter in the alphabet.
*In each major installment of the series, the final world is the thirteenth: in Kingdom Hearts, End of the World is the 13th world to be seen (counting Disney Castle); in Kingdom Hearts II, The World That Never Was is the 13th world; and in Kingdom Hearts: Birth by Sleep, it is the Keyblade Graveyard (counting Destiny Islands). In Kingdom Hearts: Chain of Memories Castle Oblivion has thirteen floors. If one counts the ground level as well, there are also thirteen basements.
*There are thirteen letters in "End of the World", "Hollow Bastion" and "Radiant Garden".
*Put together, there are thirteen letters in "Light" and "Darkness".
*In Kingdom Hearts Re:coded, there is a System Sector in Hollow Bastion that contains 13 floors.
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