Sora and Riku have been best friends since very early in the childhood. They had a friendly rivalry and always sparred together with their wooden swords yet they still had a great admiration for each other. Sora and Riku both shared the desire to travel to other worlds and this especially became the case after Kairi arrived. They decided to build a raft together but were eventually sucked away from Destiny Islands by the darkness.
As soon as Sora and Riku ended up in separate worlds, they wanted to find each other. They eventually ran into each in Traverse Town. Sora was happy to see him but Donald said that Riku could not join them so he ran away. Maleficent tried to convince Riku that Sora did not care about him anymore and he seemed to believe this. Riku began to side with the heartless which angered Sora. They fought multiple times in Hollow Bastion and Riku temporarily stole Sora's keyblade until he overpowered his heart and got it back.
Riku was upset that Sora's heart overpowered his and allowed Ansem to possess him. Sora eventually figured this out and demanded that he gave Riku his heart back. While the closing the Door to Darkness, they saw each other again. Riku helped him close the door and told him to take care of Kairi. Sora decided to continue to look for Riku. He went to Castle Oblivion and thought that he found him but it turned out that it was just a replica of Riku.
Sora was eventually put into a coma as a result of having his memories tampered with. Riku was determined to help him wake up and learned about his nobody Roxas. He fought Roxas in an attempt to capture him but could not win. Riku decided to give into the darkness in order to win which transformed his appearance into Xehanort's heartless and temporarily caused his memories to fade. He watched over Roxas until he returned himself to Sora.
Since Riku had given in to the darkness and changed his appearance, he did not want Sora to find him. While Sora was traveling, he dropped him some clues in order to help him move on. Sora and Riku eventually found each other in the World that Never Was and had a heart-warming reunion. They were eventually the last ones there and defeated Xemnas together. Sora and Riku were trapped in the Realm of Darkness together and had a heart-to-heart conversation. Riku admitted that he had always been jealous of Sora but the best thing about him was having him for a friend. The Door to Light eventually opened and they returned to the Destiny Islands together.
Sora and Riku spent some time at home until they received a letter from King Mickey. They were both called to take the Mark of Mastery exam by Master Yen Sid. Sora traveled through the Realm of Sleep while he was supposed to while Riku was actually traveling through Sora's dreams. They were determined to find each other and their actions still affected each other. It was eventually revealed that Xehanort intended to use Sora as a vessel and Riku had to save him. Afterwards, Yen Sid named Riku a keyblade master and Sora was really excited for him.
Soriku is a very popular Kingdom Hearts ship and is possibly the most popular ship of them all. A lot of fans shipped this over SoKai from the start. However, this ship especially became very popular after Kingdom Hearts II. This was the first game where there was no ill feelings between the two and their relationship was emphasized more. Many fans feel that there is no way that the two are just friends.