Sora and Kairi knew each other from living in the Destiny Islands. Kairi ended up in that world from Radiant Garden and Sora decided to go meet her when she was living with the mayor. They eventually became good friends and always spent time together along with Riku. Being a princess of heart, Kairi was pure and acted as Sora's light.
Eventually, Sora and Kairi decided to try and travel to other worlds along with Riku on a raft. However, the heartless attacked the world. Kairi lost her heart which sought refuge inside of Sora's heart. As soon as Sora was taken from the world and ended up in Traverse Town, he was determined to find her. He hallucinated seeing her at multiple points in time. He saw Yuffie as her and thought that he saw her in Deep Jungle and Merlin's hideout.
Sora eventually saw Kairi's body in a comatose state. At Hollow Bastion, Riku who was possessed by Ansem revealed that Kairi's heart was inside of Sora and attempted to steal it. Sora eventually released his heart to return Kairi's heart to her and was turned into a heartless. Kairi embraced Sora which allowed him to return to his normal self. Afterwards, she gave Sora her lucky charm and they decided that they would always be in each other's hearts. After Sora defeated Ansem, Kairi was returned to Destiny Islands while Sora went to look for Riku. Kairi would wait patiently for Sora every day.
Sora eventually had his memories changed and Kairi replaced with Naminé. Because of this, he had to go into a coma in order to have his memories fixed. During this period of time, most of those who where connected to Sora forgot about him. However, while Kairi forgot about his name and face, she still remembered Sora's existence. Roxas eventually connected to Kairi telepathically which in turn connected her to Sora. This allowed her to remember his name. She wrote him a letter in a bottle and placed it into the ocean.
Sora eventually woke up and traveled the worlds to search for Riku. While waiting for him to return, Kairi was captured by Organization XIII. Sora eventually learned about his and was devastated. He even got onto his knees and begged Saïx to take him to her. Kairi was eventually rescued and the two were reunited in the World that Never Was. They immediately embraced.
At one point, Kairi was returned to Destiny Islands while Sora and Riku were left behind to fight Xemnas. They ended up being stuck in the Realm of Darkness. However, Sora received Kairi's letter and this allowed him to open the Door to Light. He and Riku were returned to Destiny Islands and he returned Kairi's lucky charm to her. However, Sora was eventually called to do more work so Kairi gave him her lucky charm again and told him to hurry back.
SoKai is rather popular Kingdom Hearts ship. It is sometimes overshadowed by slash pairings such as SoRiku but is one of the most popular het pairings. This pairing is often regarded as being semi-canon due to the romantic references and the fact that the two were deeply connected through their hearts. Many fans found the ending of the first game to be sad since the two were separated and were happy when they finally got a sequel in which they were reunited.
- In Kingdom Hearts II, it was implied that Sora has feelings for Kairi. He blushed when he saw Will and Elizabeth together and Donald and Goofy mentioned that he was thinking about her. When he saw Jack and Sally dancing, he imagined himself dancing with her in their place. Roxas also mentioned that Kairi is the girl that Sora likes.