Disclaimer: One Piece and all the characters belong to Eiichiro Oda. I don't make a profit off of this.
A/N: Based on my headcanon I posted on tumblr. Yes, I cried while writing that headcanon, and yes, I cried while writing this(a little bit).
Luffy doesn't think about Ace often, but when he does, he sits on the head of Thousand Sunny and looks out at the ocean. After a while, he'll pull a bead from his pocket and will just look at it. It's a red bead that looks old, and altogether ordinary. It's dirty, and scratched, but that's okay. It belonged to Ace, and that's all that matters to Luffy. It's a part of Ace that Luffy brings with him everywhere. Luffy thinks that if he brings the bead with him, the day he becomes King of the Pirates, Ace will be there too.
For now, Luffy holds the bead, thinking that Ace is watching over him, cheering him on. He knows Ace would be proud of him. That doesn't stop the tears though. They don't come often, but every once in awhile, when he's missing Ace, he'll cry. Luffy always misses Ace, the pain of losing his precious brother never goes away, but he resolves to move forward. He doesn't look back on what he could have done, or what could have been, he just misses Ace. He misses knowing that Ace is somewhere out there in the world, doing whatever he does. The ache in his heart doesn't fade, but when he's with nakama, he can forget about it for a little while.
On days like these, Sanji will make him lots of extra meat, Usopp will tell him the funniest stories, and Chopper will want to play a lot more. Zoro would sit quietly by the figurehead and wait for Luffy to come down. Nami and Robin will give Luffy the space he needs. Franky will have something cool to show him, and Brook will play cheerful music, or join him in melancholy and play 'Bink's Sake'. Each of his nakama has their own way of trying to cheer him up, and Luffy appreciates it. He appreciates the crew being there for him.
Today is a crying day, but Luffy resolves that he won't cry. He bites his lip and swallows the tears as he thinks of Ace. Pushing the tears away, he shoves the bead in his pocket, and wipes his eyes. Turning around to face the crew scattered around on the deck, he forces a grin and rejoins them on deck to play games.
Later that night, when everyone's asleep in bed, or out on deck on watch, Luffy pulls the blankets over his head, and cries.
Watch over me, Ace!