Basically, Ds are demons who can cloak themselves to be normal people. Animals and people with strong instincts can kind of tell unless the D really trains to be completely unnoticeable.
Lami was the one who pulled Law to hide under the bodies of their friends. She didn't even look concerned about the body fluids. Law would admit only to himself that he was scared of her in that moment. Her unhinged, feral grin and unhidden features scared him almost as much as the white patches and the marines shooting his friends in the street.
Lami was the one who pulled him to join the Donquixote pirates. "We're going to die anyway," She said with almost a chirp and Law was afraid, "Why don't we take out as much as we can."
"Okay," He agreed, because he really did want to destroy the government, but also because he couldn't say no to his little sister, even if she was scary instead of sweet now.
Lami convinced the pirates to let them join, and Law stabbed Corazon. Who didn't rat him out for breaking the blood law.
The other children pestered Law, so he told them about his name, and Corazon kidnapped him.
Lami, upon seeing her brother getting kidnapped, climbed up the side of one of the buildings and dropped onto Corazon in a kicking motion. It hurt her more than him, but it got him to startle.
Less than twenty four hours later, Corazon kidnapped Lami and Law from the Donquixote pirates and set off to find them a hospital that could and would treat them. Both tried to explain to him that it was futile.
"Hey Law," Lami said while they were supposed to be sleeping, "What if he does find a cure? What do you want to do?"
Law turned toward her, "I wanted to be a doctor."
"Do you still want that?" Lami asked, her features shifting between humanoid and demonic.
"... I don't know," Law said, "I still want to finish my studies."
"I can't imagine feeling safe trapped on land anymore," Lami admitted quietly, her fangs digging into her lower lip. Law pulled her close and gently pried her lip out of her mouth.
"Then we'll sail," Law said, "and we'll be free from the government's control."
"Yeah," Lami said, "that sounds good."
If Corazon heard any of their conversation, he didn't show it. He kept taking them to hospitals and then getting turned away and then he'd leave them somewhere and come back covered in ash and smelling of smoke that was certainly not cigarette smoke.
The sicker the children got, the less they could control their appearance, and eventually, it was impossible to appear human at all, so Cora-san couldn't take them to hospitals anymore.
Cora-san called them adorable and ran his fingers through their hair. He also told them his idea for a cure since, at the sight of either child, most doctors would either run in fear or get weapons out.
Lami, in her semi-delirious state, had nudged Cora-san's hand with her head and started purring when he absentmindedly stroked her hair while explaining. "Nii-san should eat the fruit," Lami said, "I'm not good at doctor-y things."
"That's not a word," Law chimed in immediately.
"See?" Lami said, "He's smart."
"Right," Cora-san said, "He's going to be a doctor, right?"
"A pirate," Law said quietly.
"Pirate captain doctor, sir," Lami said and giggled.
Cora-san got Law the devil fruit, and Lami chirped at him.
"I don't speak chirps, Lami," Cora-san said.
"Run," Lami said, "Danger."
Law didn't even stop to think before dragging Lami to her feet and trying to do the same for Cora-san. "Which direction?" Law asked.
"What?" Cora-san asked.
Lami pointed away from shore. "Go that way. Caves," she said and trailed off in a long hiss-like sound.
"Lami did this before, right before the marines came to Flevance," Law said, "Now get up, let's go."
Cora-san got up and picked up both children before bolting in the direction Lami pointed out. Miraculously, he didn't trip. Lami made weird clicking noises that Law found mildly terrifying, but they were going to live through this.
They didn't actually find caves on minion, but they did find a way off of the island that they took and ended up on Swallow, where they did find caves. Law and Lami were both exhausted and fell asleep the moment they were someplace warm enough to not die immediately.
Cora-san put his feathered coat over the two of them and tried to go out to get food. Lami grabbed his wrist in her sleep, though, "Stay?"
"I'll stay," He said, and sat on the side Law wasn't on.
It took less than a week for Law to figure out his first technique and use it to remove some of the poison from Lami and himself. Cora-san was impressed. It was hard to learn to use such a complicated fruit.
Lami, upon being fully healed, shifted from her demonic form to her human form and laughed. Her laugh used to be childish, but now it sounded other. Law privately thought it was an excellent laugh for his first mate. He knew his laugh would probably sound mildly evil now too.
"It's weird to not be in pain," Lami whispered to Law when Cora-san was out of earshot, "Thank you, Nii-san."
Law smiled at her, and Cora-san came back with water for both of them. They drank it greedily.
Lami started blending into shadows when Cora-san and Law weren't watching. When they looked at her, half-merged with the shadows, she would grin with too many teeth and step out of the shadows while shifting back to human. Law wanted to see what somebody who wasn't insane would think upon seeing it. Cora-san tended to grin like an idiot.
Law stopped hiding his features fully. His teeth always stayed just a bit too sharp and his eyes a bit too bright. Lami, on the other hand, started hiding her demonic features to such a precise degree that, if Law didn't know any better, he'd say she was a perfectly ordinary human. Even Penguin and Shachi, who were crew and not properly Ds seemed less human than her. Cora-san called it Uncanny Valley when he visited.
He also praised her and taught her to be a spy, so Law was even more scared of her than ever.
"My instincts don't even warn me anymore," Bepo said when she walked in, "They warned me about both of you when we met."
"Really?" Lami asked, "How long ago did they stop?"
"Uh," Bepo said, "Maybe a week? I'm sorry for not being more precise."
"I just appreciate knowing at all," Lami said, "I'll have to test this further… That is if you're okay with it."
"You're kind of scarier when you aren't scary," Bepo said.
"Should I stop?" Lami asked, "I don't want to make you uncomfortable."
"No," Bepo declined, "It's probably a good skill."
Lami had taken Cora-san's spy training to heart and enjoyed breaking into marine bases to steal information and destroy documents. She also started compiling evidence of illegal dealings from various marine bases. More bases were corrupt than not. Lami started leaving copies of parts of her evidence in the uncorrupt bases.
Law started taking patients and ever so casually breaking the law to do so. He stole medicine from marine bases, beat up thugs and marines alike, and kicked asshole doctors into hospital beds, all the while teaching his crew how to play nurse and or doctor whenever they were needed.
Bepo became the navigator and also the best besides Law at treating broken bones. Shachi became the mechanic for their sub and became the best besides Law at stitches. Penguin became the chef and the best besides Law at treating diseases. Lami became the spy and assassin, so she learned how to treat poisons as well as make and use them.
Bepo was the best of all of them at being Law's nurse. Lami said it was because he was telepathic, which he denied.
"Hey, Law, at some point today, will you call for Bepo without actually calling for him?" Lami asked while they were picking herbs and the rest of the crew was in town.
"You mean with gestures or with my mind?" Law asked sceptically.
"With your mind," Lami said, "He doesn't think he's telepathic."
"Is he telepathic?" Law asked.
"Only one way to find out," Lami said.
Law indulged his little sister two hours later when he realized he forgot to make sure they got operation room 2 restocked before they left the island. Bepo appeared in the doorway fifteen seconds later, "Yes, captain?"
"Holy shit," Law said, "Lami was right."
"What?" Bepo asked.
"It slipped my mind during the marine ambush, but is operation room 2 fully stocked?" Law asked.
"Penguin and Shachi worked together to restock based on your notes of stock," Bepo reported, "I'm not sure whether or not they were able to fully restock."
"Thank you."
"What was Lami right about?" Bepo asked.
"Oh, that you're telepathic," Law said, "I didn't actually say a word to call you over."
Bepo stared at him blankly.
"Penguin owes me five thousand Beli," Lami sang from the doorway, her demonic form barely cloaked at all.
"But I'm not telepathic?" Bepo argued.
(Author's Note) Sorry, Bepo, I make the rules here. You're telepathic.
Ds each have a minor power that will not really be explained.
I didn't really want to change Law's backstory too much, but obviously since Lami is alive, things are different.