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This is written from when Luffy was younger, after Ace left the island when they were younger.
Spoilers ahead!
Dear Shanks,
It's been awhile Shanks! Ace just left to become a pirate today, but told me I couldn't go with him. I'm really sad about it, but I'll get my own crew that's just as good as his! I still have three more years before that happens though… I still have your hat, but it always falls off my head. I need a string or something for it.
I've gotten a lot stronger! I can take you on anytime you want to come back here! But just so you know, I still won't ask you if I can join your crew, because I have to become the Pirate King! That's the deal we made, right?
Makino talks about how she misses you all the time, but I've heard around the village that there is a guy who is interested in her, but I haven't seen him. You should really come back soon, otherwise you might lose your chance!
Ace always talked about wanting to meet you, but I have no idea why. I still live on the mountains with Dadan and the group, but it's going to be lonely without Ace here to train with. But I have to keep training!
I will keep my promise of becoming the Pirate King, Just you wait!
Sincerely,
Monkey D. Luffy
P.S. Makino helped me write this, but she doesn't know that I wrote the part about that guy, but you better hurry back!
P.P.S. I miss your crew and your food.