Do I think that Aqua/Ven are lovers?
No, I don't think they are. Clearly close friends, or brother/sister sort, but I doubt they were lovers. Then again, there's still not much known about them, so who knows?
I' ve always wondered what Xemnas wanted with her though. I did see the scene of him in Hollow Bastion, and while I don't know what he said, it's clear that he was talking to her. It took place while Sora was talking to Mickey and right before the battle with Demyx.
Aqua represents Water. So does Demyx and he also controls it. Goofy is knocked out after Demyx is defeated and Sora, of course, freaks out, but is forced to leave. What happened in the meantime? What happened while Sora cried out to Goofy? I've said to before and I've said it again, things like that don't happen in stories for no good reason. We already know that the situation is dangerous, so there had to be another reason. Some KH fans theorize that it was to prevent Roxas from awakening after Demyx's death. Others? Well, I don't know since it's not often talked about.
Demyx is the "center" of it all, so to speak. I mean, to me, the game is divided into "Before Demyx" and "After Demyx". The defining battle of the game(1000 Heartless), Xemnas's appearance and Sora's first meeting with Axel since COM happened AFTER Demyx.
But, why would Xemnas speak with Aqua BEFORE the fight with Demyx? What did he want with her? I refuse to believe that Demyx was just a "distraction", especially considering that the conversation with Aqua took place right "Before Demyx".
I just want to believe that Demyx was more than just minor "comic relief".