WARNING: Idon't know anyone with DID. I tried my best, please enjoy!
Ace didn't really know when it all started. One day, he was suddenly aware that there was something inside him. Two something. At the very beginning, they were just voices in his head that told him things, like to hit people when they said that Gold Roger's son shouldn't exist or that it was true; he shouldn't have been born. He thought that it was normal to feel that way. After all, it wasn't like he had someone to go to when all these feeling clawed at his tiny chest, making him feel vulnerable and useless and God- he hated to feel that way. His little body and his unripe psyche were crushed by the intensity of his emotions, making him struggle to even stand up on his own two feet. But he had to. He had to stand up and go hunt because no matter how young he was, no matter how crushed he felt, if he didn't work he couldn't have food. Bandit's rule. And if he didn't eat he felt even more crushed than before so he just had to take his staff and hope his strength wouldn't leave him just now.
It was in his eighth's spring that something in his precarious balance just snapped. It started with a hole in his memories. A moment he was in a bar, asking the same question as always, and the next he was in the woods, staff bloodied and breath short. It started with minutes, then it ate away entire hours. He was starting to freak out, when the first depersonalization hit him. He was frozen, a sense of dread inside his chest as he watched through his eyes action that he wasn't making. He was in a bar, asking the costumers the same damned question and when they answered him hell broke loose. He watched himself lift a table and throw it before hitting everything and everyone that was around him. This...thing wasn't him. It wasn't him whom fractured that man's jaw! It wasn't him that kicked that other man's shin so hard that the bone came out. It wasn't him, it wasn't him!
When he finally got back his ability of controlling his body, he was terrified and shaking. He was afraid of the sheer amount of blood thirst he could feel in his mind. A blood thirst that wasn't his own. He wasn't like that, he never wanted to kill. Never. It took him a lot of time to calm down, until he heard a voice in his head.
"You don't have to listen to these idiots" it said. "I will beat the shit out of them next time."
"Who are you?" He asked, scared, with a whisper. This voice was violent, but not as much as the...presence he felt before, inside his body.
"I am you. But I am also someone different from you."
"Did you...did you do that?"
"No. That was something else."
And they talked and Ace learnt that he wasn't alone in his head anymore.
He remembers returning home and asking Dadan about it, but she just yelled at him to stop talking about useless things and go hunting instead. Garp's reaction was to check his head, wondering if he hit him too hard.
It wasn't until Sabo came around that he learnt he had a Dissociative Identity Disorder, or DID. He learned that there were three people besides himself in his mind. The angry voice, that appeared whenever he didn't want to hear people talk about Roger or talk to the bandits. That personality was out like 80% of the time, only receding when they were with Sabo. He and Sabo decided that King was a good name for him. He was really angry and refused to bend, like a real king.
But there were other personalities.
One of them hated himself -and Ace- with a desperation that made him feel like drawning. They called him Diamond, because whenever he was in control, tars would steam down his cheeks, clear and shining like diamonds. Besides, Sabo thought it was a funny thing to call his personality with card-related things.
The last one, the one with that murderous intent, was Red&Black, or, like Luffy later decided, R&B. They called like that because whenever he was out, Ace would feel a black, cold blanked around his conscience and red, warm blood on his hands. It was disturbing and King was dead set on making him never surface again. Ever.
It wasn't until he met Makino and he understood how much he yearned being protected and taken care of that his mind gave birth to his fourth personality. It was a woman, the second girl Ace ever talked to, and she was the most caring and loving person he ever 'met'. She was like Makino, but her presence was warmer and she called him 'my sweet son' inside the intimacy of his crowded mind. She was named Queen before Ace could even think about a name for her. But it didn't bothered him, it suited her after all. She didn't show herself, Queen seemed content to just talk to him sweetly and lovingly, and Ace was happy. King too, in the period he lived happily with his brothers, receded into a voice inside his mind. He never felt so complete before he met Luffy and become brother with him and Sabo. His personalities could feel his calmness, becoming tranquil inside of him and Ace soon enough was used to the light buzz of whispered conversation in his head.
But, of course, all good thing came to an end. Before Ace could comprehend that Sabo wasn't with them anymore, he was dead. Gone forever. And he was left to deal with a crying little brother and a gaping hole in his chest. It hurt like hell and felt like he couldn't breath because of the pressure. Ace was sure he was going mad, until, one morning, he woke up with Sabo's personality inside of him.
He remembered waking up because of his exited voice in his ear.
"Ace, wake up!"
Spade was...just like Sabo. Intelligent, curious and with a deep hatred for the nobles. He helped with the hole 'raising Luffy alone' ordeal. During the time he was in control, Spade would read and read and read and write. Whenever Ace was in control and had difficulties dealing with Luffy, he would whisper in his ear, his witty remarks soothing him.
In a blink, seven years passed and he, alone with his five personalities -six if you counted Ace himself-, found himself on a boat stuffed full in the middle of the sea.