After the marriage of Todoroki Enji and his wife, came the birth of their first son, Natsu. He had the same fiery red hair as his father, and the sweet temperament of his mother.
When Natsu was two, his mother gave birth to a second son, Yukio, who had his mother's hair and father's eyes.
When Natsu was three, Toya was born, with his father's hair and mother's eyes.
When Natsu was four, he developed his Quirk, called Fire Sculpture. He could use the fire around him and manipulate it to his own desires. The day after that, he began to 'train' with his father. His mother tried to stop his father, but was unsuccessful, and ended up getting backhanded.
When Natsu was six, his little sister Fuyumi was born, with white hair streaked in red. Yukio gained his Quirk, as well, called Snowshift: He could manipulate water molecules in the air to form snow, then use that snow to his fancy. His father, [luckily, in Natsu's opinion] considered the ability to be too weak, and was not put into training like Natsu. Toya, on the other hand, was not so lucky. He gained his quirk within a week of Yukio's; the ability to create fire from his body. It only output fire tot he point his body could handle it, though. Natsu taught Toya how to hide his fire, saying that if their father found out what Toya could do, he would hurt him.
Miko was born after two years, but died three days after birth. Natsu didn't see his mother for months after that.
Toya successfully hid his powers for three years, but when Shouto was born, his hair lit up in little blue flames. After that day, he was forced to train with Natsu, who was now eight, and tried to stop his father in every way he could, including straight-up taking the blows for Toya. Their father got angry at this, and would lock Natsu into a Quirk-proof room when training with Toya to prevent this.
That training continued for five years, until Shouto showed his Quirk. By that time, Natsu was being prepped in how to attend UA, a prestigious high school, for which he would be getting into on recommendation. Shouto's Quirk was a perfect mix between both of his parents', with ice coming out of his left half and fire out of his right. Their mother had always doted especially on Shouto, Yukio, and Fuyumi, but became even more so on Shouto once his Quirk manifested. She would sit for an hour with him before every one of his training sessions, watching All Might cartoons.
A year later, he left for UA, living in a nearby apartment. Not a month in, he was given a letter from his father, stating that his mother had burned his brother with a hot tea kettle, and would be sent to a psychiatric hospital.
His first two years at UA after that passed without incident, and Natsu made quite a few friends.
The last day of his second year, during graduation, he got a call from Yukio, telling Natsu of the terrible news: When Toya had tried to stop an eight-year old Shouto's training, he was forced past the limit of fire that his body could handle, causing him to burn himself alive. He was considered dead, and had seemingly been burnt to complete ash. He was told the ceremony was in two weeks, and that Endeavor was going to make a speech.
For the first time in his seventeen year old life, Natsu held complete and utter hatred for the man who had sired him. He absolutely hated the man. Despised him.
When he saw his brothers and sister two weeks later, in the back row of a church, he could tell the rest of them agreed. Shouto, at age eight, Fuyumi, at eleven, Yukio, at sixteen, and he himself, at seventeen, all knew that they hated the bastard who had murdered their dear brother and drove their mother to insanity. They all wanted him dead.
The day Natsu graduated from UA, he moved to Tokyo, and created a small hero agency with his trust fund. He worked odd jobs on the side, and stockpiled his money. He became the not well known hero Firebrand, and protected a small downtown area dense with small shops. He offered Yukio a job there, as soon as Yukio was able to move out of the house, and Yukio took it once Shouto turned eleven. By that time, Fuyumi was fifteen, and could take care of Shouto better.
When Shouto was fourteen, he disappeared. Fuyumi moved out the day Shouto left, and took her trust fund with her, as well as her little brother's stuff. She got a job as an accountant for Natsu's hero business. Natsu, Yukio, and Fuyumi lived together, looking for Shouto as best they could with their combined funds. When it was heard that the League of Villains, a recent villain group, had gained a member who could use fire, the three thought maybe Shouto had decided to be a villain to spite their father. They cheered for him, from the sidelines.
At about the same time, a man, about nineteen or twenty, had begun to hang around the hero agency, in the park across the street. He had dark reddish-black hair, and was severely burned on his arms, neck, and underneath his eyes. The burnt skin had seemed to be held together with stitches to the healthy parts of his body. One day, Yukio got curious, and went over to investigate. His report afterward was that the man seemed very friendly, nice, and held no malicious intent, but had said something rather odd. He had greeted Yukio by name, and then asked him why he had waited so long, since he was usually the most curious.
The man had introduced himself as Dabi, and said no other odd things, except once mentioning 'to my family, I'm dead', and 'my little brother is fifteen'.
The three siblings thought nothing of it, and life continued on. Occasionally, they would see another black haired boy with Dabi, as well as a green haired boy who seemed to be the boy's friend. The green haired boy had a white cane in one of his hands, but seldom actually used it. He seemed to have rather sharp ears, and Natsu often wondered if the boy had an Echolocation quirk, because he seemed to be able to navigate very well despite being blind. Or, Natsu suspected he was blind, with the white cane and the way he looked near, but not at, the person(s) who was speaking.
Then, all at once, all three of them disappeared. Natsu had marked nothing unusual in the days preceding it, besides an offhand comment Fuyumi made about her wondering how Shouto was doing. The next day, the three had seemed tense, but had relaxed the day after. Three days later, it seemed that they'd never existed. A search of 'Dabi' through all known databases only had two hits:
Cremation, and the known member of the League of Villains, who could use blue fire and hated the pro hero, Endeavor.
That's when everything fell into place for Natsu.
Dabi- 'Cremation', the hatred for Endeavor, the discomfort at the mention of Shouto, the aversion to talking about family, the comment about Yukio, how his family thought he was dead, how he knew their names, his lack of his own last name, his 'little brother'...
Dabi was their lost brother, Toya, who they believed was dead, burned to death, cremated, he was there, alive, and they had no clue.
And the 'brother', who had disliked talking and was always so secretive, didn't give them a name other than 'Netsu-shima', the aversion to touch and his initial reaction to Natsu's red hair... it was Shouto.
The green haired blind boy, that was the only mystery.
One that was easily solved, when the League attacked Endeavor's hero agency.
Then, three villains made their most public début.
Two with black hair, one with green.
The green haired boy called himself 'Kingpin', the leader of the League.
One of the black haired boys, with a scar on his face and equal control of ice and fire, called himself Torment.
The second one with black hair, with burns along his neck, arms, and under his eyes, called himself Dabi.
The two black haired men completely destroyed the forty-floor building, with almost everyone still inside. The green haired one watched, sitting on a throne of ice and debris, as the number two hero's legacy burned to the ground.
Surprisingly enough, the wreckage didn't crush any of the innocents on the nearby sidewalk, conveniently placed ice and cars sheltered small children and their parents.
The only nearby civilians who were killed were Kai Chisaki and Chizome Akaguro, who, after investigation, were two wanted criminals.
Kai Chisaki was the leader of the Eight Precepts of Death, codename Overhaul, and Chizome Akaguro was the hero killer known as Stain.
Two days later, Dabi showed up at the park again, with his little brother and the mysterious green haired boy.
The siblings went out to greet them as usual, and Natsu gave Dabi a hug.
Yukio followed suit, as did Fuyumi, and the three of them hugged Shouto as well.
"We're okay with it. We don't mind. If you need help, you know where to find your siblings." they reassured their family.
The mysterious boy finally spoke. "My name's Izuku. Sorry for not speaking earlier. I'm... I'm known as Kingpin."
"Nice to meet you." Natsu greeted.
Izuku tugged on Shouto's sleeve, and Shouto grimaced.
"I'm... dating Izuku."
"That's great! Shouto, if you hurt this boi I will kill you, villain or no." Fuyumi warned, hugging them both.
"Ignore her, I'll be the one to kill you." Yukio interjected.
"Guys. Pls." Izuku whined. "I'd kill him myself."
"Ooh, kinky." another voice drops in.
"Shut up, Toga." Dabi shouted, throwing his backpack into a nearby tree.
There was a muffled 'oof', and then a crash as a girl in a high school uniform crashed out of a tree, Dabi's backpack clutched to her chest. "Fuckhead."
"Dicklover." Dabi returned.
"You're one to talk." Izuku muttered.
"What?"
"Like ur not gay af."
"Shut up!" Dabi screeched.
"Wait, Dabi's gay too! I forgot!" Toga cheered, dusting off her skirt. "I remember, I tried to flirt and you just left me alone. I was so sad, I accidentally stabbed Izuku."
"Izuku remembers." Izuku glumly whispered, shuddering.
"Shouto does too. He had to thaw me out of the ice." Toga recalled.
"Okay then." Yukio not so subtly moved so he was behind his little sister, away from Toga. "How about we all go over to our house and have tea, and maybe some cinnamon rolls?"
"I'm down to watch some cannibalism." Shouto remarked offhandedly. "Let's go."
"Wait, cannibalism?" Yukio asked.
"Yay! Cinnamon rolls!" Toga cheered.
"Sure, I love cinnamon rolls." Dabi agreed.
"Is anyone going to answer me? Why did Shouto say cannibalism?"
"Let's go!" Shouto said louder.
"Yay! Cinn-i-mon-rolls, cinn-i-mon-rolls!" Izuku began to chant, and soon the others joined in as well, and began to leave.
"I don't understand, why did Shouto say there'd be cannibalism?!" Yukio questioned, tears beginning to form at the stupidity.
"Just... just don't question it." Dabi advised. "Much better that way."
Yukio sighed, a long, suffering sigh, one that encompassed all of his pain into a single exhale.
"Fuck it. Cinnamon rolls and cannibalism. Why not."