Birth by Sleep Ventus and Xion are the most similar.

WARNING! THIS THREAD IS SPOILER HEAVY!

Everyone seems to state how Ventus is similar to Sora & Roxas, and he is. But you may find this weird, but Ventus (in my opinion) is most similar to Xion.

If you think about both of their stories, it kind of makes sense:

-Unlike Sora, Ventus didn't have a love scenario with Aqua (or at least not in a boyfriend-girlfriend way). This was similar with Xion. Xion didn't have a love interest with Roxas either.

-Both of their stories are really sad

-Obviously, both are Keyblade weilders

-Also, there is a part in 358 where Xion appears as Ventus in front of Xigbar. This could be hinting that they have some type of connection.

-Both of them "Died"*

-Both of them had to be destroyed/killed. (Ventus-->XBlade/Xion-->Sora's Memory)

See????/????Slash?

*Neither of them actually died. Both fused with Sora, and became part of him

 

The thing about Xion appearing as Ven to Xigbar, that hints at a connection between Ven and Sora. Xion is a memory clone of Sora, so any connection she has to anyone is the same connection that Sora has.