It's too late...for me to undo my mistakes. But you can't let Xemnas...have Kingdom Hearts. You can't. Good-bye, Roxas. See you again. I'm glad...I got to meet you. Oh...and of course, Axel, too. You're both my best friends. Never forget. That's the truth.
~ Xion, before she fades away.
Roxas, don't be sad. I came from you and Sora. I am you...the same way that I'm Sora. You'll forget me, but the memories themselves will never go away. Memories of you and me will always be together...forever inside him.
~ Xion's final words to Roxas before he forgets her completely.
Xion is a number of the Organization XIII and one of the main protagonists of the Kingdom Hearts series. She resembles Kairi due to the fact that she is Sora's most precious and powerful memory.
First arriving at Organization XIII, Xion is a shy, secluded individual. When going on a mission with Roxas, Xion begins to open up to him and later Axel. She does seem to express emotions due to her nature of existence, and is always happy to be with her friends. However, she is somewhat emotionally fragile, becoming deeply enraged with the right provocation. After finding out the origins of her existence, she also becomes deeply confused and lost. She is observant and perceptive. She has a strong sense of justice and generosity, seen in the way she willingly gave up herself to join with Sora. She resembles Kairi and Naminé in the way that she fights to protect those she has come to care for.
Xion's Organization coat is similar to the one Roxas wears, with slightly pointed shoulders and closer-fitting sleeves. She also wears the more feminine, heeled boots that Larxene wears and the basic black gloves. Physically, Xion strongly resembles Kairi in her appearance from Kingdom Hearts, having her blue eyes and virtually the same haircut, though Xion's hair is black and her bangs part on a different side. She is slightly shorter than Roxas. As a result of her being crafted from Sora's memories, Xion's appearance is somewhat unstable, as she is known to transform into Roxas (in his Black Cloak) and both Riku and Sora in their original appearances from the first game.
Her appearance is also slightly different depending on who is looking at her, and their relation to her. Most of the Organization, such as Saïx and, early on, Axel, view her as a hooded puppet, while to Xigbar, she appears identical to Ventus. Xemnas, on the other hand, sees her true form—a perfect copy of Sora. In one of his secret reports, Axel states that Xion looks like Naminé, although it is not clear if he truly saw her as Naminé at that time, or if he simply was referring to the physical similarities that Xion and Naminé share. An interesting note is that once Axel considers Xion a friend, from that point forward, he only sees her as the girl that she is instead of a hooded puppet.
Tomoco Kanemaki is responsible for Xion having black hair.
When Xion begins to absorb Sora's memories from each of the devices that were placed in Wonderland, Halloween Town, and Agrabah, she gains armor that causes her to resemble a mechanized version of Sora as he appeared in Kingdom Hearts. Her armor changes based on the amount of memories that she has taken in, though some features stay the same. Her initial armor has the form of a pink and white helmet that has spikes on the top, sides, and back. Her body armor is a black, white, and pink jacket with black and pink gloves. The bottom half of the armor consists of mainly black and gold, with golden and white shoes. If one pays attention, her silhouette matches that of Sora.
During her first form in the Bizarre Room in Wonderland, she uses a pair of green, white, and pink wings, and carries a golden and pink blade. In her second form in Halloween Town, she uses a large, long blade inspired by Halloween Town itself, and uses the Ragnarok limit. While in her Agrabah form, she appears to have four arms, as well as having additional golden shoulder pads and purple and white blades, for the Ars Arcanum attack. Finally, her final form in Twilight Town is a large, upscaled version of the original, with two pink lances in the shape of the Nobody symbol. This last form slightly resembles Sora as he looks in Kingdom Hearts II.
Xion was heavily implied to be the final member of the real Organization XIII based on the Final Battle Trailer for Kingdom Hearts III, given Lea didn't know anyone other than Roxas who could wield the Kingdom Key; this turned out to be true, although she ended up defecting from the True Organization and joining the Guardians of Light instead when she regained her memories of Roxas and Axel.
The artwork included with the Stained Glass #6 [EX+] medal in Kingdom Hearts Unchained χ / Union χ