Question: why does xigbar see xion as ventus?
Answer: Xigbar saw her as Ventus, most likely due to her links to Sora, and therefore, Ven. ... Roxas, who is a part of Sora, sees Xion as a black haired KH1 Kairi because she is composed of Sora's memories of Kairi. Axel becomes Roxas and Xion's close friend and thus begins to see her in the same way.