#but I can't cite a specific point in-canon to indicate they have a RELATIONSHIP

[getvalentined]

Getting back on my Zack Is Canonically Polyamorous soapbox because the people need to know and also the number of people accusing Aerith of being unfaithful in Rebirth is absolutely fucking ridiculous when taking Zack's romantic escapades into account.

There are spoilers here, and I'm tagging accordingly, but I'm not putting it under a cut.

We know for a fact that Zack is in a relationship with Aerith. We've known this since the OG, although we didn't know most of the details back then, just that they were together. In Remake, Aerith describes Zack as her "first love." In Chapter 11 of Rebirth, the first memories of Zack that Cloud reclaims are of him talking about how great Aerith is, and Cloud describes him as being "head over heels" for her. In Chapter 8, we found out that Zack's parents were able to recognize her based entirely on Zack gushing about her when he wrote home—and this is one of the most important parts, honestly.

Zack has a thing with Cissnei. We don't know how deep it is, but throughout Crisis Core (and even in the summer event in Ever Crisis) we can see very clearly that there is something between them, and it's never treated like a big deal. She's never completely honest about her feelings, so we can assume he didn't know that she saw him as anything more than a casual thing, but they have a relationship. It's made explicitly clear that this is the case when Zack returns to Gongaga and she lets him know that she's spoken to his parents, and he warns her that they're basically going to suggest that she marry him.

Zack's parents know about his relationship with Aerith and they also apparently know enough about his relationship with Cissnei to suggest that she make that relationship "official." They want her to join the family. They asked, she apparently reciprocated, and she lives in Gongaga after Veld is "killed."

Cissnei is also aware that Zack is in a relationship with Aerith, but that doesn't stop her from pursuing him anyway. She gets it and she's okay with it. She never, at any point, tries to push him away from Aerith—she's the one that tells him how special Aerith is, how important, how lonely. She's supportive of his relationship, and doesn't seem to feel that it makes her own feelings or their relationship any less valid.

Okay, so that's proof that the people around him are, at the very least, okay with him having multiple partners. That doesn't necessarily mean he's poly, though, maybe he's just a womanizer and everyone accepts it. Weird, but possible. He's really endearing so maybe they all give him a pass.

But no! No this isn't it at all! And we know it because of how Zack responds to Aerith being in love with someone else in Rebirth. When he finds out that Aerith is has feelings for Cloud, there's not a hint of jealousy in a single thing he does or says. He's disappointed when Marlene says it's because he wasn't there, but it's not disappointment in Aerith or in Cloud—it's disappointment in himself, because Marlene is right. He wasn't there. He couldn't protect her. There's also no spite there, he never responds like her feelings are wrong because he's here now and that means Aerith can rely on him again, and we know that because of how he responds to this information.

Zack finds out that Aerith has feelings for Cloud, and his immediate mission is to make sure that Cloud recovers as quickly as possible. Now it's not just for him, it's not just because that's his friend, it's because Aerith loves him (too) and that means he has to save Cloud for both of them.

He's fully supportive of Aerith and Cloud having a relationship, to the point that the last thing he says to Cloud when he gets thrown into a dying timeline is a request that he take care of Aerith. He's not exactly passing the torch here, because he's still alive—but he doesn't know if he'll be able to meet back up with a version of Cloud that is awake again, and he needs Cloud to know that he supports them in this.

And then, in the dying timeline where Zack is banished after fighting Sephiroth, he has the ability to synergize with Cloud from across the multiverse. The DMW limit break connected to Aerith is active in the space. All three of them are together here, just not physically; Cloud and Aerith are protecting Zack as much as Zack has been struggling to protect them.

Zack loves that Aerith loves someone other than him because that's how he is too. Zack has multiple relationships at the same time, he loves multiple people at the same time, he has emotionally intimate connections with multiple people at the same time. Why would he take issue with the love of his life feeling the same way?

Cloud isn't his replacement any more than Cissnei and Aerith are interchangeable, and Zack understands that, because Zack is polyamorous—and he's really happy that Aerith may be, too.