Is she that important for the story tho? I think she and MC are moved on and stuff, plus some already decided who they are dating(for the rest of the game?) and we have a lot of unfinished romance options for season 3.
People tend to think she's going to fuck the MC up mentally and make him question himself and the relationship he's in with one (or many) of the other girls. She'll interject herself somewhere in the MC's life. Maybe join the HOTs? Who really knows, but she's bound to do something.
Zoey is the mc's first love:
mc: "I guess I really didn't see her as a girlfriend until that point when we said our goodbyes to each other."
mc: "It actually hurt...a lot."
mc: "But once again, I wasn't completely sure if I was sad for losing my best friend...or for losing something more."
They hung out a lot. For years.
The mc knows Zoey better than he knows any of the new girls (even if he stalked Josy for a few months prior to actually talking to her).
So yeah, Zoey is significant in his life. Sure, they parted ways, and lost touch since the last 6 months or so. But it makes sense that he'd still care about her if she showed up on his doorstep (or his dad's doorstep as the case may be).
That doesn't mean he'd lose all self control and start fucking her right after he confessed his love to Sage/Bella/Jill/M&J (take your pick), but it doesn't mean he'd kick her to the kerb either. But if he wanted to fuck her, it'd be nice if the offer was on the table, you know, for old time's sake.
This is the mc's story, and she's a part of it. But DPC couldn't start the story with her.
If he started the story with the mc in junior high, getting beaten up, learning to fight, beating up his aggressors, ending up in detention, meeting Zoey, her being his (girl)friend in junior high (and keeping their relationship PG), then reaching senior high, and somehow they are both 18+, then they start fucking,
and then she fucking leaves?! Everyone would have gone ape shit.
"Why didn't you just burn her to death in a fire DPC, did you learn nothing from AL's fallout, you still wanna fuck with our hearts you cruel bastard!"
No, that wouldn't have been smart. (He waited until episode 4 before he started fucking with the readers, less chance of everyone bailing if they are already well and truly hooked.)
Instead we (the reader) "met her" when there were already other girls on the table (when he shares his past with Bella), so her leaving the mc in the past doesn't hit the reader like it hit the mc.
But now it's time to actually meet her. That's where the Interlude comes in. It'll probably go back to the beginning. Quickly catch up to when they were 18+ and then they have their sexual awakening. The mc (i.e. us) gets to make a few choices while they were together, perhaps shaping how they resume their relationship on her return (was it love, were they just great friends with benefits?)
And then the story follows Zoey, off to San Diego to hit the waves. They keep in touch (maybe we even flash back to the mc messaging her from time-to-time), and then shit hits the fan, life for Zoey isn't so great, she realises her new boyfriend's cock is too small and shrivelled, no way she wants that thing in her!
So she packs her bags and heads back to where she knows the cock is prime.
But, maybe the mc has just declared his undying love for Jill, what now? Well Neil sure seemed happy to see her, so hopefully the apple didn't fall far from the tree.
Ok, that last sentence was a joke, but the rest is entirely feasible.