Such wilt thou be to me. There are four main female characters Zack is involved with: Yuffie, Cissnei, Aeris, and Tifa. I won't bother with Yuffie, as she was a little kid. Nor will I mention Tifa, just because she seems mostly disinterested in Zack and after being injured by Sephiroth, curses SOLDIER. Now, we just have Cissnei and Aeris. Before anyone decides to read the riot act to me, please remember, these are my opinion.
Aeris
Before leaving for Nibelheim, knowing that it could be a lengthy mission, Zack asks Tseng to watch over her. He also asks the Angeal copy to do the same. I think because the game is Japanese, that the subtle romantic implications may be missing, as their is a huge difference between how one goes about expressing their feelings in Japanese society and American society. I think this is proven when Aeris calls Zack during his Nibelheim mission. She'd obviously tried several time, as when he does answer she responds that Finally my call came through!. Zack has to work and must make the call brief, so he tells her to call back. Aeris lets him a call back isn't necessary, and after a moment of silence between them Zack responds "I understand. I'll come see you". She does not question when he'll return, only "I'll be waiting". I understand. That simple statement would be out of place, if Zack felt nothing but friendship for her, but that simple phrase speaks volumes - the same way she longs to see him again, he wants to see her as well.
When Zack escapes from the Shinra basement, and finds the paper on which Aeris wrote her one wish, he is determined to make it to Midgar. Midgar is probably the worst place he could go. With Shinra hunting him down, to return to Shinra HQ? And he is also well aware the Turks have Aeris under surveillance . Someone is probably expectinghim to come back to the church. The smartest thing to do would be to get as far from Midgar and stay low. But all Zack can think of is Aeris' wish and seeing her once again. It's irrationally, but for him, she is the only thing rational left in the world - Angeal is goe, Sephiroth is mad, Shinra and all he believed is a lie.
No one writes 89 letters if they don't care deeply for the person. Aeris waited four years for Zack to return before giving up on him. Aeris has 23 hopes, but the only one she writes down and gives to Zack is for the two of them to meet again. When Zack dies, she is aware someone close to her has passed, and after seemingly consulting the Planet, seemingly realizes that person was Zack. As Zack had suggested to her, she makes a living selling flowers in the slum.
A funny contradiction is, at the end of Crisis Core, there is no doubt Aeris sensed Zack's death. She looked to the sky and was clearly shaken and in disbelief...then she prays. However, in FFVII the suggestion is she has no idea what happens to Zack and thinks he's found someone else. Perhaps this was just an error on Square's part, or Aeris in denial. Who's to say...
She never once left the slums - that is until meets someone with the same sky blue, Mako eyes, Cloud Strife. She immediately notes Zack's likeness in Cloud. She readily admits to Cloud that he reminds her of a SOLDIER she dated, taking him to the same playground she and Zack ended their date. But Cloud is not Zack, if you went on a date with Aeris in gold Saucer, she admits this. Zack is obviously someone she hasn't gotten over, mostly because she told him she'd be waiting. She never got any closure. So she never removes the ribbon Zack gave her. She even uses the ribbon to attach the only item she had that belonged to her real mother, Holy. Only when Aeris dies, does both the ribbon and Holy fall away.
One's first love stays with you. The innocence of it, the feelings. Zack and Aeris genuinely enjoyed each others company and spoke with actions, words, and sometimes, in volumes, with silence. Aeris saw Cloud's Mako eyes and saw so many similarities in his personality to Zack -whom he claimed he never met- that she had to have know something was up with him. Even Tifa knew something was askew with Cloud, mostly because she has known him in childhood and knew that the things he recalled were not the truth. We have to remember that Aeris is a bit more in touch with things, things that can't be seen or felt by most, because she is a Cetra. I think Aeris' feelings towards Cloud was more nurturing that love- she wanted to help him through whatever was wrong with him. Help him because of the memories of Zack he invoked..
As Zack is dying, the final DMW he has is of Aeris, in which she says to him "I'll be waiting". Zack's final memories remain on Aeris before his DMW fails him completely, probably because he knows that her dearest wish was to for them to meet again - and so was his. He's dying. He knows he's not going to make it. He knows he won't be able to go to her. So he just remembers...
Finally, in Loveless, there is a quote: Though no oath is shared between the lovers, In their hearts they know they will meet again. Zack never promises Aeris a thing, nor does Aeris ask for any promises. Though they are never together in life again, but they are reunited once more in the LifeStream. In Advent Children, the two are together, and return to the LifeStream together after they are sure that Cloud will be alright alone.
Final verdict: I absolutely see no love interest between Aeris and Cloud. I, for one, would not want anyone dating me because I remind them of their dead ex (awkward much?) and more importantly Aeris is no 'homie hopper'.
Cissnei
She does seem to go out of her way for Zack at times. A friend would tell her boss that she hadn't found you when you are standing right in front of her. A good friend would give you the keys to a motorcycle to make a get away. However, someone who is interested would go to your home and discuss you with you parents. She's a bit younger than Zack though. Reno and Rude search for Zack and Cloud because Tseng ordered them, however, Cissnei and Tseng seems to have more of a personal interest in Zack's welfare, as they continue their desperate search to find Zack first.
When talking with Zack's mother in Gongaga, Zack's mom expresses her concern whether or not Zack will be able to find a wife now, to which it seems Cissnei implied that there was more to her and Zack's relationship. That or she only told Zack she made that implication to get him flustered.