1) spoiler mark your comment :\ not everyone who visits my page finished the game.
2)
Spoilers
They explained as much in the game itself. Xion looks different to anyone who looks at her, depending on what they can relate to in the Memories she has that belonged to Sora. Namine is Kairi's Nobody and knows Xion was based on Sora's Memories of Kairi, so she sees Kairi, as does Riku. Xigbar couldn't care less about Kairi and probably Namine, but as Braig, he saw TAV. Namely, Ven would be his point of familiarity in Sora's Memories. Hence, he saw Xion as Ven.