Xion was created as a replica of Roxas with some of Sora's leaked memories. As such, they had a connection before they even met. Xion was originally created with the purpose of absorbing Roxas so that Sora would never wake up and that Organization XIII would always have a keyblade wielder. Xion would eventually encounter and battle Xion and it turned out that she actually appeared as Sora to him.
Xion was receiving Sora's memories and eventually began to wonder about them so she went off to seek the truth. She eventually learned that she would never to give herself up and be forgotten in order for Sora to be able to wake up. She left the organization but was eventually brought back by Axel. Xion then began to absorb Sora's memories at a faster rate.
Xion eventually met Naminé and learned more about Sora's memories and the effects that they were having on her and Roxas. She was eventually taken back into the organization again and "reset" by Xemnas. Sora's face began to manifest on her as she fought Roxas. She lost and she was fading away, mentioned that they were both Sora and would always be together. Roxas was determined to free her and was going to attack the organization so she had Riku stop him so that Sora could wake up.
While Xion was forgotten, she still remained deeply connected to Sora. When Aqua was asking Ansem about the new keyblade wielder in the Realm of Darkness, Xion was seen on the clock tower in Twilight Town with Axel and Roxas and said Sora's name, as if she was calling out for him. As Sora was wandering through the sleeping worlds, he ended up in the World that Never Was and had a vision of Naminé. She turned into Xion and Sora shed a tear. Sora was eventually stuck unconscious again so Riku had to go rescue him. Xion appeared inside of Sora's heart and asked him what his wish was. Riku answering her question allowed Sora to wake up.
SoXion has some popularity in the Kingdom Hearts fandom. It is not as popular as other Xion ships such as RokuShi or Rion and other Sora ships such as SoKai and Soriku greatly overshadow it. However, this ship still does have its fans. It mostly has fans due to the fact that Xion looks like Kairi and the fact that Sora cried a little bit when he finally got to meet her. With the secret ending of Birth by Sleep showing Xion and Sora mentioning that others were still sad, some hope that Sora will eventually be able to bring Xion back.
- Xion took an appearance similar to Kairi because she was Sora's strongest memory.