Princesses of Heart

The seven Princesses of Heart are maidens devoid of darkness in their hearts. If they are all brought together the Door to Darkness will open. Princesses who lose their hearts do not become Heartless, due to their distinctive lack of darkness.

Members
The seven princesses of heart are: With the exception of Alice, and the game's new addition, Kairi, the Princesses of Heart are also a part of the Disney Princess franchise. Conversely, Ariel and Mulan, two members of the Disney Princesses who both appear in the Kingdom Hearts series, are not considered Princesses of Heart in the games. Pocahontas is the only official Disney Princess who has yet to appear in the Kingdom Hearts franchise.
 * Kairi
 * Snow White (from "Snow White and the Seven Dwarfs")
 * Cinderella (from "Cinderella")
 * Alice (from "Alice in Wonderland")
 * Aurora (from "Sleeping Beauty")
 * Belle (from "Beauty and the Beast")
 * Jasmine (from "Aladdin")

Kingdom Hearts
In order to open the Final Keyhole, Maleficent, the Heartless and Riku kidnapped each of the princesses from their respective worlds. During this kidnapping spree, Riku went to Neverland with Captain Hook under the belief that Wendy was the final Princess of Heart. However, through unknown means, Maleficient was able to reveal that this assumption was false and that Kairi was actually the final princess. But because Kairi had given her heart to Sora, the power of the princesses' hearts could not be fully utilized. Thus, Xehanort's Heartless used the hearts of the remaining six princesses to create the Dark Keyblade, which could release Kairi's heart from inside Sora. However, Sora stabbed himself with the keyblade to revive the princesses. Once they had escaped their imprisonment, the Princesses quickly worked together to stop the flow of darkness that had begun to seep from Hollow Bastion's keyhole. Once Sora sealed the keyhole, the princesses patiently awaited Sora's victory so that they could return home, which they finally did when Kingdom Hearts was sealed and the worlds restored.

Kingdom Hearts: Chain of Memories
Alice, Jasmine, Belle and Kairi all appeared in Kingdom Hearts: Chain of Memories as figments of Sora memories. Also, by remembering Kairi, Sora could start regaining his lost memories. In Riku's story Naminé appears as Kairi to tell Riku not to hide from the darkness in his heart.

Kingdom Hearts II
The seven Princesses did not have large roles in Kingdom Hearts II. Belle and Jasmine played prominent roles in their respective worlds. In Tron's world, Space Paranoids, the password to the DTD Dataspace was the names of the princesses. Said password, according to Sora, Donald and Goofy was: Belle, Snow White, Aurora, Alice, Jasmine, Cinderella, Kairi (in password form, likely entered with no spaces or commas). Kairi was the princess with the largest role; she was captured by Organization XIII in their attempt to fuel Sora's anger, thus causing him to kill more Heartless and release more hearts for Kingdom Hearts' reconstruction.

Trivia

 * In the Awakening world, there are five glass pillars, four of which depict four Princesses of Heart (Snow White, Cinderella, Aurora and Belle). The fifth is a pink stained glass pillar showing three silhouettes of girls framed by heart shapes. Despite that these silhouettes bear a striking resemblance to Alice, it is also believed that they symbolize the three princesses who have yet to be captured by Maleficent's group, those three being Alice, Jasmine and Kairi.


 * Although the princesses have the power to battle darkness using their pure light, the way they do this has not been explicity shown, although it is possible that their presence simply does the trick, or that they call upon light similar to how Minnie Mouse did in Kingdom Hearts II.


 * Through pictures released in gaming magazines, it has been implied that the princesses who have not had their worlds featured in the games thus far (Snow White, Cinderella and Aurora) will have their stories told in Kingdom Hearts: Birth by Sleep, which takes place ten years before the events of Kingdom Hearts.


 * Wonderland has been revealed as a world in Kingdom Hearts 358/2 Days, implying that Alice will make her third apperance in the series.


 * Before the identities of all the Princesses of Heart were revealed, Ariel was easily mistaken to be one of them, because of her Disney Princess status.


 * Wendy was mistaken to be one of the seven Princesses of Heart by Captain Hook, but Riku told him that she wasn't.


 * Though this is not confirmed (as Kairi has shown to have aged twice), there is a possibility that the Princesses of Heart can actually remain youthful, as Cinderella, Aurora and Snow White remained young and beautiful in Kingdom Hearts, where it sets ten years after the events of Birth By Sleep. It is more likely to say that Maleficent kept them in suspended state, preserving their youth.