Friends of Olde
Kairi is the primary female supporting character of Kingdom Hearts, and remains as one of the most important people to Sora. For a large portion of the game, Kairi's heart disappeared, leaving her unconscious. Unbeknownst to Sora, her heart had been trapped inside his own since their departure from Destiny Islands. Her relationship with Sora reaches a much deeper level as they are each other's love interest. Perhaps the most important event of this relationship was at Hollow Bastion, when Sora turned into a heartless. Because of Kairi, he was able to transform back into a human, a rare ability that only a few people manage to accomplish. Her strong will compensates for her physical weakness, stemming from her caring nature. Towards the end, she and Sora decide to keep a promise: that when they meet again, Sora would return the Oathkeeper to Kairi. Sora refers to this keychain as his good luck charm, and would play a key element in his determination to reunite with Kairi someday.
After her departure with Sora at the end of Kingdom Hearts, she was left at her home on Destiny Islands while Riku and Sora were away. Kairi eventually forgets Sora's name, but suddenly remembers it when her memories confront her of Sora's existence along with Sora's voice. When captured and held prisoner in The World That Never Was, she is rescued by Riku, and given her own keyblade to fight with. She finally reunited with Sora, and later runs to greet him upon their return to Destiny Islands at the end of the game.
Riku is both Sora's rival and closest friend. He is the first person that Sora loses to the darkness; Riku's wanderlust and determination to venture off to new lands is driven by his fearlessness of the dark. When Riku learns of the new friends Sora has made during his travels, he grews jealous, which turned into hatred, thus allowing his heart to grow weak. Riku misunderstands the situation and makes the assumption that Sora would abandon him and Kairi for them, and eventually turns against him. Riku seems to be easily possessed and persuaded, which becomes his ultimate downfall. However, in the end, he was able to relieve himself from the clutches of darkness, and risked his heart in order to keep the heartless from invading. Before he could return, however, he needed to seek redemption for what he did. As punishment, he was unable to return to his original form without blindfolding himself.
By the second game, Riku was still in the process of atoning for his actions, allying himself with DiZ and King Mickey. He asked King Mickey to not reveal his identity to Sora as he was ashamed for he put him through in the past. When he finally finds Sora in Kingdom Hearts II, he is in a form unrecognizable to him. By the end of the game, Sora and Riku leave the world together, agreeing to protect the world of light by being the darkness.
Sora's relationship with Riku has been a peculiar one since their childhood days; though Riku and Sora are the best of friends, they maintained a healthy rivalry. It wasn't until Riku and Sora left Destiny Islands when their relationship began to take a turn. Riku had turned to the darkness, leaving Sora worriedly chasing after him.