To quote Vexen...
Basement ten of Kingdom Hearts: Chain of Memories - Reverse/Rebirth:
"Sora and Riku's existence may be interesting..."
Note the wording. It's not their presence that Vexen finds interesting, not the fact that they're in the castle, not their strengths or personalities, necessarily. (Though, the reason he prefers Riku is due to the darkness and the whole I'll take the road to dawn thing.) No, what interests Vexen is their very existence. Why would the nobodies care about the life of some random person (currently ignoring the keyblade- and darkness-wielder details)? For Sora, well, consider this. Why do the members of Org13 want to open kingdom Hearts? Because they want hearts. They want to be human, presumably to turn back into their Others. Sora exists. Roxas exists. This is likely something they had not seen before. That is why Sora intrigues them, because it should not be possible for the Other to live while the Nobody does. Now, this is my favorite part, Vexen grouped Riku in with Sora.
Earlier in that scene, when Zexion identifies the intruder as Riku, Vexen seems surprised...
Zex: I've identified the scent. It's Riku.
Vex: Riku? How could he have escaped from the realm of darkness?
Zex: He once shouldered the darkness.
Vex: I see. Because he was so close to that side. The dark power given to him facilitated his escape from the realm of darkness.
I'm pretty damn sure I'm reading too much into this, but just his mention of Riku being a captive of the darkness... When Riku gave himself to the darkness... Again, Vexen could have thought (likely along with the rest of Org13) is that Riku, the "Other," was lost to "Ansem," and he could be commenting on the Other's return while Axel his Nobody lives... If you read them carefully and in the right frame of mind, the clues all line up a bit too perfectly, yeah...?
... And I'd like to thank Yami for inadvertently pointing this out to me when she commented on Vexen's evil laugh.
"Sora and Riku's existence may be interesting..."
Note the wording. It's not their presence that Vexen finds interesting, not the fact that they're in the castle, not their strengths or personalities, necessarily. (Though, the reason he prefers Riku is due to the darkness and the whole I'll take the road to dawn thing.) No, what interests Vexen is their very existence. Why would the nobodies care about the life of some random person (currently ignoring the keyblade- and darkness-wielder details)? For Sora, well, consider this. Why do the members of Org13 want to open kingdom Hearts? Because they want hearts. They want to be human, presumably to turn back into their Others. Sora exists. Roxas exists. This is likely something they had not seen before. That is why Sora intrigues them, because it should not be possible for the Other to live while the Nobody does. Now, this is my favorite part, Vexen grouped Riku in with Sora.
Earlier in that scene, when Zexion identifies the intruder as Riku, Vexen seems surprised...
Zex: I've identified the scent. It's Riku.
Vex: Riku? How could he have escaped from the realm of darkness?
Zex: He once shouldered the darkness.
Vex: I see. Because he was so close to that side. The dark power given to him facilitated his escape from the realm of darkness.
I'm pretty damn sure I'm reading too much into this, but just his mention of Riku being a captive of the darkness... When Riku gave himself to the darkness... Again, Vexen could have thought (likely along with the rest of Org13) is that Riku, the "Other," was lost to "Ansem," and he could be commenting on the Other's return while Axel his Nobody lives... If you read them carefully and in the right frame of mind, the clues all line up a bit too perfectly, yeah...?
... And I'd like to thank Yami for inadvertently pointing this out to me when she commented on Vexen's evil laugh.