Oops! All Yanderes! Strawhats where they’re all mutually obsessed with each other is peak but even better is when they also share each other’s obsessions with non-crew-members.
Koby. I am talking about Koby.
Luffy didn’t force Koby to join his crew because being a marine is his dream, so he let him go, but it aches and chews holes in him and the rest of the crew very quickly catch on that their captain is pining for a darling somewhere out there. It’s Zoro who ends up explaining the little pink-haired marine who was so scared and so brave and would have been so very easy to snatch up.
Luffy has given the Strawhats everything, freedom and home and family and a path toward their dreams, and he demands everything from them in payment. He’s demanding and selfish and spoiled, as greedy as any self-respecting pirate should be — but not about this. This one thing he wants so very badly, this one thing he did not take. So, naturally, his Nakama will just need to get it for him.
Every time the Strawhats run into Koby, Luffy has to white-knuckle his fists to keep himself in check, but the others have absolutely no qualms about it. They get into a fight with Koby’s unit and decimate the whole squad but nobody puts even a single scratch on Koby. They coo and fawn at him about it, remind him that they would never, ever hurt him. They say he should really consider switching sides. They would never pit him against an opponent they knew he couldn’t beat. They would never send him into battle with so little backup. They’d take care of him.
Koby gets almost used to the touching. They are all constantly touching him. Fingers in his hair, on his face, arms around his shoulders and waist, hands… hands on his hips, on his thighs. In his hair, always, Koby feels like he can't go outside without being backed against a wall by a pirate who seems to want nothing more than to ask how he's been and if he needs anything while running their fingers through his hair.
Koby realizes he’s being stalked. He can’t go more than a month without running into the Strawhats, and they’re blatant about it — when confronted, they outright say they missed him and came to see him. He says it’s insane they’re willing to risk drawing the navy’s attention like that just to come harass him. They all laugh, tell him they aren’t scared of the navy, and he shouldn’t be scared of them. Not ever. Koby should never, ever be afraid of them. Is he ready to come home now? They’ve all been waiting so long.
Koby transfers again. Keeps his blinds closed and stays away from windows. He tries to shave his hair, but a lead star breaks through the window and knocks the clippers out of his hands. The next time the Strawhats come for him, Usopp runs possessive fingers through Koby’s pretty pink hair. Tells him to never do that again. Luffy talks so much about how he loves Koby’s hair, and they wouldn’t want to upset their captain, right?
Koby needs to be perfect. He’s a gift for Luffy, a sign of appreciation from the crew. He can be stubborn, but he is not allowed to do anything that might ruin the gift.
Maybe the Koby's superior officers eventually cotton onto the strange way the Strawhats treat him. Maybe Garp figures out that something is wrong, and tries to hide Koby away. Maybe someone decides to use Koby as bait to lure Luffy and the crew into a trap. Maybe they assume Koby has ties to the Strawhats, and arrest him on the chance of treason. Maybe a lot of things.
Of course, how it happens doesn't change the ending: Koby, no longer a marine officer. Not ever again. A pirate now, like he should have been from the very start, home and with his nakama where he belongs. Presented to Luffy with a pretty bow around his neck as a gift to his captain who loves him so very much.
Luffy, tearful with gratitude, bounces around giving everyone hugs and thanking them profusely. This is the nicest present anyone has ever given him. He demands a feast to celebrate, of course, and as Sanji serves up dinner Koby is seated in Luffy's lap with one of Luffy's arms wrapped around and around his waist to keep him there.