Ace knew, ever since he could remember, that he was different. It was a gut feeling, an instinctive sense of wrongness that would bubble up as he watched the others. He was different and different was who he was.
It was in the ways people - the bandits usually - would look at him as he grew up. Grew up too fast and polite for a child living in the jungle with bandits as his caretakers. Dadan noticed it, Sabo noticed it, and even the perpetually oblivious Garp noticed it. Which said a lot about Ace's subtlety.
The only person who paid it no mind was Luffy. Since even his ignorance knew no bounds. But that didn't bother Ace. He was different and different was who he was.
The stares first came when Ace spelled out his name without being taught how to read or write. Then when he spoke with a vocabulary of a mature adult of at least ten years his senior. Then when he came back one day covered in scratches and bleeding, dragging home a dead boar three times his size. No one questioned Ace, but instead they stared. For that was all they could do without labeling him as a genius. If they did, Garp would surely expect more from them, and they had no more to give the little boy other than clothes and a roof over his head.
Ace grew used to these questioning stares and carried on as if nothing mattered. To him the bandits were akin to a small child prodding a sleeping bear with a stick, curiously hoping to see what would happen if the bear woke. He was amused by their antics and paid them no mind. He was different and different was who he was.
So when Garp broke the news to him that Gol D. Roger was his father, Ace was not surprised. He did not explode in anger and dive deep into the never ending pit of self-hatred as he'd previously done. (Look where that had led him. To a premature and gruesome death was where it was.)
Ace merely nodded and asked Garp if he could have another pair of shoes, his were getting too small. Garp, expecting more of a reaction from Ace, looked at him with a dumbfounded expression. But he quickly recovered and informed Ace that he would buy the growing boy another set of clothes, shoes included.
Ace smiled and thanked the older man, asking if Garp would be staying over for dinner. Again Garp was stumped by the boy and could only hastily agree. Ace shot him a grin and informed everyone that he would go hunting for food for dinner and would be back soon. At his quick departure, Garp took a step to follow, afraid that Ace would be injured, or worse injure another person if he were to unleash his emotions at taking in the big news out of their supervision. However, Ace turned and put up a hand to motion for Garp to stop. Then he adorned another small grin and said,
"Think what you want shitty gramps, but I don't really care if he's my father. After all, I've got you, don't I?"
Garp's jaw dropped and Ace smirked at the man's facial expression. Then the freckled boy ran into the jungle, leaving only a breeze behind.
Garp was frozen for quite a while until Dadan reached for his collar and tried to shake some sense into him. (Something not even his parents had quite accomplished, judging by Garp's "logic.")
Dadan cursed him for bringing such an important and wanted brat into the jungle and entrusting him to the bandits. The bandits wailed and sobbed into each other's shoulders and handkerchiefs, pulled out of thin air just for the occasion, and lamented their fate at housing Ace. Garp merely flopped back and forth. What seemed to be his soul tried to slither out of his body in shock. Though unsuccessfully as Dadan grasped it and pushed it roughly down his throat.
If Garp shed a few tears at the boy's words, no one mentioned it.