the one where you can taste anything your soulmate tastes (food, blood, dirt, anything in the mouth)
Ace learned early on that his soulmate didn't eat breakfast. It was a "maybe once a week" meal, usually pretty sparse on carbs. Fruits Ace had never seen would burst on his tongue. Sometimes a nutty crack between teeth that stuck to his gums. Rarely bread. But always coffee.
Ace hated coffee. At 4 years old he'd cry angry hot tears at the bitter taste and put anything and everything nearby in his mouth to erase the taste. His own breakfast was lingered over, every chewing bite drawn out to drown the flavor of coffee. Water never helped. He chewed Dadan's table and plates and chair backs hoping the taste of varnish and wood would erase the bitter aftertaste. Dirt, leather shoe soles, knife oil, the grease from the bottom of the skillet-nothing escaped Ace's attempts to get rid of the taste of coffee in the morning.
Luckily, his soulmate got the hint. Coffee never stopped being a staple of his soulmate's mornings, but the taste changed. Cream, sugar, milk, different blends that diluted and dulled it to nearly unnoticeable levels. Ace stopped trying to literally eat Dadan's house and life moved on.
Ace could almost always forgive the coffee by lunch time. For all his soulmate seemed to hate breakfast, lunch (and dinner) were always a treat. On days Ace couldn't scrounge up his own meals aside from whatever roughage he and Sabo could scrounge up, his soulmate somehow always had a feast. Flavors he couldn't identify lit up his tongue without fail-spices that complimented meats he'd never known, creamy textures of soups and smooth pastas or mashed roots and tubers, slick slides of bitter-but-not-coffee-and-not-bad-at-all drinks drew pictures in his mind of better meals. Desserts-sweets unlike anything he'd imagined-comforted him at night, every night, without fail. For a period of time he was nearly sad that his soulmate had to make due with his meager meals of foraged greens and unseasoned meat.
Eventually, he figured that one day-once he'd set sail and started his own journey on the seas-he'd make it up to his soulmate and eat every fancy meal he could find in every port he visited. It was a promise he kept to himself-his spare money that wasn't spent on the crew or sent home to Makino and Dadan went right to his stomach.
When he joined Whitebeard's crew his attitude about it didn't change. He kind of regretted starving himself and stealing naked breads and fruits from the kitchens, but his soulmate seemed undisturbed by the change. There were even moments when the smells wafting up to his hiding places from the mess on the Moby reminded him heavily of meals his soulmate was eating or had eaten in the past. Even the conspicuously left-out-in-the-open desserts that the daft cook left unprotected in the kitchens at night were the same sometimes.
Yet, somehow, he didn't put the dots together until that fateful day Marco told him to make up his mind. After the right hand of Whitebeard left him to his thoughts, he'd picked up the bowl of soup and knocked it back. Then he'd nearly spat it all over himself and the deck once he registered the taste.
The exact same thing his own soulmate had eaten barely an hour before. His soulmate was on the Moby Dick. His soulmate was a member of Whitebeard's crew. Hell, it was possible Ace had fucking met them already and hadn't known it.
He'd be a liar if he ever said that finally knowing who preferred coffee over actually eating in the morning wasn't a factor in his final choice.
It still took him weeks to figure out, after he took Pops' mark.