Well Xion was Roxas' Replica, and she was only created during Roxas' first week of existence. During that time, Roxas was blank slate, having little to no conscious memories of his own, and having not even developed his own personality. If the method used to create the Riku Replica was applied when making Xion, then she would be based off the data of her "original", this being Roxas. Since Roxas was a blank slate back then, she too took form as a blank slate, but in her case it affected Xion's appearance as well. She, like Roxas, was formed with no memories or personality. It was only after sufficient contact with Roxas that she was able to copy Roxas' powers, including Roxas' ability to absorb Sora's memories and power. And from then on, Xion subconsciously redirected Sora's memories to flow to her through Roxas.
Again though, since she was based off Roxas when he was just a blank slate, she herself appears as a blank slate to those who view her but are not connected to one of the memories she contains. When she gained enough of Sora and Roxas' memories though, she began to look different to numerous characters. The way a person saw her largely depended on whether the person was connected to one of the memories she contained. If so, then that person would see Xion as someone in the most prominent memory that the individual was most connected to.