Birth by Sleep connection to 358/2 Days (Possible Spoilers)

There is a mission where Xion, Axel, and Xigbar are paired together in Wonderland.

But Xion and Xigbar get into a Fight, Why do you think Xigbar sees Xion as Ven?
That scene had me completely confused....I know everyone sees Xion differently, but why do you think Xigbar sees Ven?

 

This has been answered many times.
The real reason is still unknown.
But everyone sees Xion differently depending on their connection to Sora.
Braig (Xigbar before becoming a Nobody) saw Terra, Ven, and Aqua with their Keyblades.
So Ven was the only connection Xigbar had to Sora and he saw Xion as Ven.
But just how Ven is connected to Sora, no one knows yet.