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It's interesting that people mention Crisis Core here, but completely ignore Zack and Cloud's conversation concerning Tifa. Zack badgers Cloud about talking to Tifa and Cloud shies away from it repeatedly, saying that he'd be too embarrased. Zack looks out for Cloud, and by extension, looks out for the woman his friend loves.

As for Zack, he's definitely fixated on Aerith since after he and Cloud escape from Nibelheim, he finds a letter from Aerith (the 84th letter apparently) which basically has her saying goodbye. He tells his buddy that he "has to get back to Midgar".

In the original game, Zack mentions Aerith in the brief flashback of Cloud's as well, but it was somewhat less than romantic sounding at the time. "I've got a place we can go to in Midgar. But wait, the mother's there too..."

But this isn't a Zack/Aerith thread, so back to the matter at hand. In your examples from Crisis Core, you totally undermined yourself becuase you continually stated that Tifa was asking about Cloud and though Zack showed concern for Tifa, as I stated above, it was only out of respect to Cloud.