ALL of them are based on other FF characters, except maybe for Roxas, but not just the Turks...
Xemnas-I think Kuja. I don't want it to be Kuja, because Kuja utterly enchants me. But I've never played FFIX, so...
Xigbar-I don't know. NO clue whatsoever.
Xaldin-Lulu, from FFX. I know it sounds odd, but think about it.
Vexen-Hojo, from FFVII. I have always known this; I assume other people know as well. The Japanese VA for Hojo is also the VA for Vexen. And Paul Eiding would make an awesome Vexen.
Lexaeus-I don't know. I feel like I should know, but I don't.
Zexion-Vincent Valentine, from FFVII. Young Vincent from Dirge of Cerberus looks EXACTLY like Zexion. Go see for yourself.
Saix-Some say Seymour. I say Tseng. He looks more like Seymour. He acts nothing like Seymour. He acts like Tseng.
Axel-Reno, as has been discussed.
Demyx-Zell, from FFVIII. This is the one that people know the most after Axel/Reno. It's kind of obvious.
Luxord-Again, I ought to know, but I don't.
Marluxia-I would say Aerith, but she's already in KH. Drives me bananas, but I don't know...maybe he's the Kuja-based one...
Larxene-Quistis, from FFVIII. I don't know why I think that. I just do. She acts more like Quistis than Elena.
Roxas-Rufus Shin-Ra, from FFVII. That's just my personal opinion. I doubt it's true. I used it as a dramatic irony once.
That's what I think.