Aerith knowing Cloud is a mercenary, who is not familiar with the Sector 6 slums (before they ever set foot in the Sector 6 slums), who cannot fall in love with her and even if he did, it would not be real, tells me she at least has some quite specific data on certain not unimportant subject.
Aerith was almost certainly able to meet with Zack's spirit after he departed – exactly like she's been able to do with others. Even if she doesn't consciously speak to him, the Cetra's connection to the Lifestream is all about them gaining knowledge from it. That means that it's more than likely she'd learn things that were important to Zack right around the moments he died which would include that:
- Zack passed the Buster Sword on to Cloud
- Cloud exists as Zack's living legacy
- They were headed for Midgar & Cloud doesn't know it well
- They were going to try to find work as Mercenaries
Especially if she's aware of the bits of Cloud's memory that are trapped in the Lifestream from his Mako Poisoning, she'd have even more to operate from. Just with the knowledge from Zack right around the moments of his death gives literally
all of context that she'd need to be able to put all of those pieces together the second she sees Cloud.
Even without having to have any sort of precognitive abilities, and literally only the ability to talk to or get knowledge of the desires of the spirits in the Lifestream like she's always had, this is all possible without any precognitive knowledge whatsoever.
Also, when Cloud asks Aerith if she knows who Sehp is. He says that he has a feeling that he isn't dead, and it zooms in on Aerith's mouth as she says "Oh, right" in an almost annoyed tone of voice. She knows something, or at least did before killing fate.
Again – we all know that Sephiroth's body was thrown into the Lifestream, but it won't return to the Planet, and now it's just chilling in the massive nexus of the flow of the Lifestream up in the Northern Crater absorbing knowledge. This is something that she'd
absolutely be aware of solely with her ability to speak with spirits in the Lifestream.
This is all how Aerith knows extra things that other people don't, and she's learned how to fake only being aware of the sort of things that should be common knowledge, and just kind of use her cheery ditzy attitude to sort of breeze past those moments. She was described by Elmyra as a chatterbox, but we know that as soon as Tseng shows up, she starts trying to hide her abilities.
This is all well what we should be expecting from OG Aerith given her stated abilities. If we're also adding in the existence of the Whispers here and the existence of Fate & Destiny, that is also portrayed as something Aerith interacts with as the Will of the Planet, but she's afraid of interacting with them, because she loses things when interacting with them, rather than gaining things like she does from the Lifestream. That presents the whole framework of her feeling comforted by being nudged along a path, but also afraid of it because it's extra information from the Planet that's hidden from her.
I think that the whole thing about the Whispers is Sephiroth trying to get access to that knowledge they're hiding by force (hence how we see that information about the future leaking out of the Harbinger and flashes of it in their minds during the attacks against it in Chapter 18. It's also why we see those same little flashes when Cloud is struggling with incursions to his mind from Sephiroth. I think that Sephiroth is the one trying to hold all the cards as a way to prod Cloud's motivations to his own benefit.
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