If Kairi was not going to be a Keyblade wielder for the light in KHIII, why give her a Keyblade to begin with? Not to mention they spent 3 different games setting up this whole thing: KHII, she uses a Keyblade completely out of the blue. BBS, they explain how she is able to use a Keyblade i.e. touching Aqua's Keyblade. DDD, they use the Secret Ending to solidify her status as a future Keyblade wielder. The
Secret Ending. Giving the whole Secret Ending over to revealing Kairi's future as a Keyblade wielder gives that revelation some pretty impressive clout, clout that just has to transfer over to what will happen for Kairi in KHIII. I mean, in KHIII Yen Sid goes to the trouble of training her just so she can do what exactly? Sit on the sidelines? For me, the whole Secret Ending is the final nail that gives Kairi a signifigance that can only be explained if she is one of the 7. Besides, they need more than just one girl (Aqua) fighting for the light side, don'cha agree?
Now, having said that, there are other girls that have been mentioned as candidates for the light side i.e. Namine and Xion. Both are a tough sell for me. Roxas is the only Nobody to wield a Keyblade, and that was probably due to having Ven's heart inside him. It wasn't only because he was the Nobody of a Keyblade wielder, otherwise Xemnas would've been able to wield one as well. You need a heart to wield a Keyblade. A heart that Namine does not have/need, now that she has reunited with Kairi. As for Xion, well, she is a robot, and her Keyblade is just a replica. As Riku said, it's a sham. Worthless. I can see Xion coming back to life to play some pivotal role in KHIII, but using a fake Keyblade from a "fake" Keyblade wielder on as important a position as one of the 7 defenders of the light? Like I said, tough sell. Of course, this goes for Repliku as well. Even more so, as he never even had a Keyblade to begin with.
As for Terra, he's already one of MX's 13 darknesses. Even if he did manage to win his way back to the light side in time for the climactic final battle (as many people are hoping), don't forget that this battle is determined on having 13 darknesses. If Terra defects early it'll completely screw everything up, seeing there's already 1 space needing to be filled on the darkness side. No, Nomura put Terra there for a reason. So we can beat the living snot out of him.
Then he'll return to the light
So in conclusion, MY list as the 7 defenders of light are thus:
1. Sora
2. Riku
3. Mickey
4. Ventus
5. Aqua
6. Kairi
7. Axel