Good job on finally completing your CK challenge.
WHY DOES XION LOOK LIKE SORA? WHY DOES VEN LOOK LIKE ROXAS? WHY DOES VANITAS LOOK LIKE SORA?
I have a feeling that you don't actually care about these answers, but I'll answer them anyways because I'm bored.

Xion looks like Sora because (SPOILER)she's a replica of Sora.

Ven looks like Roxas because (SPOILER)Ven's soul went to Sora after BBS, or something like that.

Vanitas looks like Sora because Nomura is a lazy bum who refuses to make original character models.
KINGDOM HEARTS WIKIA
Don't use this cesspool of unmoderated trash for plot information, ever.