So Aerith definitely knows what happens in OG FFVII, her saying to Cloud that falling in love for her is not real implies his feelings are just a reflection of Zack's feelings.
I am definitely starting to wonder how much she knows.
- Aerith introduces Cloud to Reno as her bodyguard, and asks Cloud "You don't mind, do you? Bodyguard work's not too different from merc stuff, right?" When he expresses surprise at her knowing he's a mercenary, she obviously makes up her response on the spot: "Uh... I guessed! From the sword!" In the original game, he had already told her he does a little bit of everything by this point (when he introduced himself)
- When Cloud asks Aerith whether she's ever heard of Sephiroth and tells her he's got a feeling Seph is still alive, she breaks eye contact, says "Oh...right" in a suddenly serious (perhaps somber?) tone, and walks away saying "C'mon. Let's go" -- much to Cloud's confusion
- When Red XIII first appears to the others, Aerith's expression is calm while the others assume battle stances. When he then runs away, Aerith smiles, says "We need to go!" and runs after him.
When they catch up to him, she says "This child's a friend" and makes the move to reassure him that he is safe rather than it being he who offers the others that reassurance, opposite to the case in the original game. Aerith, in fact, had been frightened by Nanaki at first in the original game. Here, she doesn't once show any concern around him.
Perhaps most notably, Aerith also isn't surprised like the others that Red XIII can talk.
- In her old room in the Shin-Ra building, Aerith says "The Shinra Electric Power Company isn't the real enemy. It started with them, sure...but I promise you...there's a much bigger threat. I just want to do everything in my power to help. All of you... And the planet."
Tifa replies "Aerith. What are you not telling us?"
Aerith: "I'm lost in a maze, and...every step is taking me further from the path... Every time the Whispers touch me...I lose something. A part of myself."
For someone who was supposedly just selling flowers a couple of days ago and "didn't know" Sephiroth was alive because the news five years ago said he was dead, she suddenly knows a lot about what's going on with the world.
- Aerith seems to already know about Jenova (by name even) when the party approaches its containment cylinder. She will also later refer to it as "The source. Of everything" as the Jenova Dreamweaver battle begins.
- Before the party boards the elevator going to President Shinra's office, Aerith is the one to say "Once we get in that elevator, there's no turning back."
- Before entering the Whisper singularity, Aerith briefly stops the party from continuing to deliver a passionate and ominous series of lines:
"This is the point of no return. Destiny's crossroads. … Freedom. Boundless, terrifying freedom. Like a great, never-ending sky. … Sephiroth has to be stopped. He has to be. And that's why... I'm asking you to help me. I know that, together, we can do this. But if we do... We'll be changing more than fate itself. If we succeed...if we win...we'll be changing ourselves..."
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Could some of this just be "Because she's a Cetra"? Sure. But given there's things she seems aware of here that she wasn't aware of in the original despite being a Cetra there too ... Well, it's very interesting.