It's only in Percy Jackson Fandom where shipping anything other than the main couple seemingly warrants death sentence.
Every other fandom explores so many other ships as shipping between characters helps in finding out how far the depth of their relationship might go.
Personally, I have always loved the idea of what Perachel could have been if Rick had actually tried. Imagine:
Part 1
Percy staying up thinking about this mortal he accidentally ran his sword through; she looked furious and confused and long after its over he is left wondering if that's how his mother felt when she met his father. He wonders it ceaselessly at times.
Rachel living in fear of everything she sees, plagued by dreams and visions, and this guy who ran her through with a literal sword calling her a mortal and surprised she can even see the sword just straight up leaves, taking all the answers with him. Long after it's happened, all she's left with is a canvas filled with the sketches of a sea-green eyed guy.
Then fate connects them yet again because Percy needs her. It starts with his need to fulfill the quest and her need for answers, but the awe Percy must have felt at Rachel's courage through the whole quest despite the incessant quips from Annabeth. He is sorry then that he has dragged someone like her to her death and if that weren't enough they run into the Titan King and he knows that maybe he has doomed them all and Rachel, mortal and unreliable according to Annabeth , throws a hairbrush at the literal actual Kronos himself.
On the flip side, Rachel knows for sure that whatever happens with her visions she will always dream of the sea green eyed hero. The images are everywhere. Him fighting, him negotiating, leading, saving them so she draws and draws and hopes it stops.
It has been noted somewhere in the Fandom once that the only reason Rachel was attracted to Percy was because he introduced her to a whole new world as if that isn't reason enough, as if they need a reason. As if it's not happened before with The Sea God and the Queen among mortals.
It doesn't stop for either of them cause now Rachel knows there's a prophecy hanging over Percy's head, and Percy knows she will see its outcome. So they talk of anything but this, whatever they can because neither of them wants to see how it ends, for the world and for them.
Long before Blackjack crashes his hooves on Paul's Prius, he knows it's coming; the end of the world, and it's far too late to look back. He leaves Rachel there because he is never taking her on a mission again, Morpheus knows he has enough nightmares of something happening to her.
Rachel watches him leave as a prickling at the back of her head tells her one of them isn't returning and no matter how wrong it is, she wishes against all odds that it won't be him.
After that, Rachel has only her visions to keep her company. She has started seeing someone's past , it's not his, but if she tries hard enough, she sees him once or twice. She commits the visions to memory, immortalizing them in art.
Percy doesn't speak to her for a good while after that, not because he doesn't want to, he would do anything to speak to her instead of doing this but his life's already forfeit so he might as well save the world. But he doesn't need to speak to her; they talk best in their visions. When of present, they are always of her. He understands why he sees them, for it's necessary to know what she sees, for she can't tell him, but he's glad for the excuse of it. He gets to see her, and he stays sane.
Yup, she's certifiably insane when she gets in a helicopter to see him, but he needs to know.
He was quite prepared for it, his death and her possibly becoming the Oracle later on. He knew it would happen. He is glad in some part of him that he would die long before it comes to fruition, that he would die in a world where they were together.
It would be their shared tragedy, them fulfilling their destinies as he escapes the divine while she ties herself to them.
Rachel had prepared for the same. She could give up over men , she was certain she would never think of them again after Perseus Jackson dies; it would be her eternal mourning and if someone asks she might tell them that the Spirit of Delphi lost her favored hero to her own prophecy.
But Fate's far too cruel.
Rachel is euphoric. He won't die, HE WON'T DIE. He's NOT the hero. The implications don't set in until she is facing him in the Throne room of Olympus. She says the things she doesn't even mean so she can soften the blow. She sees the break in him in his eyes as they share a last glance instead of a last kiss.
It clicks for him after Luke dies a hero. The bittersweet pang of triumph and loss. Blackjack is gone, and she's taken him. He isn't nearly as furious over that as he is about what she is to do.
He doesn't know if the curse is broken for sure, and he definitely doesn't want her to be the test run. Does she not know visions of her ending up like May Castellan are what breaks him in his worst nightmares.
He is the one who sees her take the oath, as she breaks what's left of them. A moment before all things come crashing down, she looks at him, and he looks back. The Oracle of Delphi and The Savior of Olympus have roles to play and loving the other isn't written in fates or destiny but they share one last vision of a perfect kiss as they resign themselves to their fate for the rest of their life; Their destinies forever entwined but never joined.
....Part 2 pending
(Also going to write headcanons of just perachel things and there are many so wait up)