Okay, before I start, this AU explanation is gonna be more of a summary of the story I had in my head. Basically it's more of a prologue.

I got the idea from @Laurenssyyy's book on AO3 and another book that was deleted by the author.

Warning: this is gonna be extremely longer than the first.

___

An IzukuTodoroki AU where Izuku is Shoto's twin and part of his quirks aspect is similar to IcyHot (the original name is too long so I'm calling it IcyHot). Izuku's hair would be black with his highlights depending on his flame's color, thought the base color is red.

Izuku was known to be a naive child compared Shoto but smart as well. When he was adopted by Inko, he was calmer and had grown an amazing tolerance towards Katsuki's yelling. After the incident however, he's more of a Season 4 Shoto but visibly shows more enthusiasm than him.

Izuku and Shoto were born when Enji was still training Toya, thinking that he had no choice but to train Toya over the other "failures". However he began to soften up after the twins were born and Toya was able to become a big brother to them. Though, they children still had fear with Enji so they mostly stay away.

Izuku's quirk manifested when he was 3 years old. He was crying that Rei wasn't there and light blue flames appeared on his shoulders. The flames changed to yellow when Rei was there to comfort him. The family marveled over his powers but Toya was wary around their father as he saw a glint in his eyes.

Fortunately, Enji shook away those greedy feelings after seeing the twin's joy but at times still had the thought. They took Izuku to the doctor to know about the quirk:

Name: Izuku Todoroki

Age: 3

Quirk: Heartburn (I got the name from Kirite Ryujin)

The user can summon multi-colored flames that have different effects depending on the color. The color changes depending on the user's mood. The user is able to force the flames into the desired color but it takes more energy. This also applies when the user makes the flames not to use its effects

Current Colors:

- Yellow = Happy/Joy = brighter light without the burning part, basically a lightbulb

- Light blue = Sadness (the crying kind of sadness) = cold atmospheres to the point of making ice (just imagine flames that can shoot ice upon shooting)

- Turquoise = Calm = increase plant growth faster

- Red = Anger/Excitement = plain fire, just burning

- Grey = Emptiness/Depression/Loneliness = smokescreen

- Black = Hatred/Loathing = can burn through anything, including other fire

- White = Surprised/Startled = flashbang effect (sometimes accidently caused by sneezes)

If the user has mixed feelings, the colors can also mix but the effects can be harder to control. User is most prone to have strong emotions and can accidently use the quirk. Must learn to control emotions.

As advised by the doctor, they took Izuku to control his emotions and discovering new colors and effects along the way. Just before Shoto's 4th birthday however, the family was attacked by villains who were foolish enough to challenge Endevour for revenge. They lost Izuku during the skirmish cuz he had a fever and accidently tagged along a villain with a teleporting quirk. Toya and Enji went back to find him but when he was gone, Toya was angry and Shoto was distraught. 

After that, Enji conducted a search for him but was declared dead after 3 weeks. Toya blamed Enji for picking the villain first instead of Izuku and blamed himself for not protecting him. This lead to his numerous outbursts and at one point, he caused a forest fire and went missing. He was later declared dead (basically the same as canon).

After the second death of a family member, Natsuo blamed his father for not helping him cope and thankfully did not repeat history. Though, Endevour reverted back to his old ways and trained Shoto harshly.

In reality, when Izuku teleported with the villain, he was found out and was fought for what they should do with him. One of the went too hard and gave him a concussion which gave him amnesia. He was left in the alleyway until he was found by Inko and Hisashi Midoriya. They took him to the hospital and found out about his amnesia and he remembered nothing but his first name and birthdate.

The couple were childless so they decided to adopt him. They taught Izuku how to calm his emotions and quirk, and also discovered his ironic fear of red fire and and preference to be surrounded by plants as they help him calm down.

Izuku was homeschooled until he was 7 years old and met Katsuki. The two had some sort of one-sided friendship as Izuku disliked Katsuki's inferiority complex but the latter always knew when to shut up when Izuku's around. Izuku wasn't exactly liked or hated by everyone but he was just popular for his vibe and quirk. He was mocked for his fear of red fires but nothing bad happened until the end of the semester, he went beserk at some bullies for picking on a quirkless kid. 

At that point, the whole school learned not to mess around with Izuku. It got to the point where Katsuki challenged him to a fight and he won rarely using his quirk instead with martial arts (he took private lessons with Hisashi).

When Izuku was 10 however, thats when everything changed.

The Midoriyas were out for a family dinner until they were caught in the crossfire of a villain attack. Hisashi died protecting them but the villain was ultimately defeated when Izuku used his red and light blue fires on the villain, encasing him in a red and blue ice with both temperature effects. The scene was caught by witnesses watching

The family didn't get away unscathed: the father was dead, the child received a scar on his right side (scar buddies!!), and the mother had mental scarring and was forced into an asylum. Coincidently, she's close neighbors with Rei.

During the attack, Izuku awakened his lost memories but decided not to tell his mother yet. He went through a week of absence from school while searching up on the Todoroki family's background. After 3 months, he managed to find Shoto and stalk- ahem observe him from afar. He was horrified angered when he found out Endevour's child abuse and Shoto's refusal to use his left side. After that, he refused to get in contact with them but still looked out for his brother.

After that experience, Izuku's flames became almost permanently red and black and light blue at the mention of Inko. Some thought of it as trauma from the villain attack when in it's actually after discovering about the Todoroki's. Izuku remained like this for years and refused to accept therapy. Despite this, he frequently visits his mother twice a week (unlikeascarfaceddaddyissueboi) and on rare times his flames changed color.

Skip to the end of his third year of middle school, he applied for UA and saved Katsuki from a Sludge Villain. He drew attention to himself again as the reporter said that the boy's quirk were multi-colored flames. Endevour heard of his but then brushed it off.

Izuku gained the top score on the Entrance Exam and was placed into Class 1-A. The moment Shoto entered the class he immediately spotted Izuku from his hair. He looked familiar but shrugged it off. Izuku gained 4th on the Apprehension Test above Shoto who's in 5th place ( I don't understand how a guy who only used his ice was placed 2nd). Izuku continued to gain attention from others at lunch, especially from a few GenEd students who went to Aldera.

The USJ goes on as normal except with Izuku's very swift thinking against the water villains by using his blue flames to rapidly freeze the water and grey flames by making smokescreen. When the Sports Festival was approaching, Izuku lectured (lol) the crowd to not be so dependent on quirks as that can lead to their downfall. This was actually for Shinso who Izuku defended against some bullies.

(I forgot to mention that Izuku managed to befriend 1-B by beating his whole class hand-to-hand combat and he's the only person Monoma respects)

The day before the Sports Festival, Izuku visits his mother and tells her about his bio-family. Inko was upset that he didn't tell him sooner but understood why. They discussed on what he should do if he's confronted by them but Izuku's hesitant after learning about Endevour.

So yadda yadda yadda, the first round goes the same, Izuku's first place and in the second round he managed to get Shoto to use his fire but only briefly. During break, Shoto confronted Izuku because 1)he looks familiar, and 2)he made him break his pledge. Izuku didn't take it too well and only scolded him for letting Endevour walk all over him.

Shoto: I'm not doing his bidding!!

Izuku: Of course you aren't, but not using your left side proves that Endevour has control over  you.

Shoto: It's his-

Izuku: You're not Endevour!! Then why are you treating your power like it's his!!??

Shoto: I-I...

Izuku: You're even more foolish than I thought. You and Endevour. You and I have a more common blood than you think.

Shoto was speechless at his statement but he left before he could say anything else. Katsuki was close by listening to their conversation, realizing a different reason why Izuku's always so angry.

Flashback a few minutes ago

Katsuki was surrounded by the Bakusquad again and were talking about the contestants for third round. The topic shifted to Izuku's quirk and his red-black flames. Katsuki was actually surprised that Izuku didn't tell them his quirk. Seeing the others were confused, Katsuki told them that his quirk was called Heartburn and explained about the colored flames and its effects. Kirishima pointed out since his flames were red-black, Izuku's angry and loathing all the time. Katsuki shrugged it off that it's been like that since 10 and told them about the incident.

The class began to see Izuku in a new light as they discovered that he's been suffering his whole life. Katsuki added that Izuku barely uses his quirk cuz he likes to hide his emotions. Shoto was close by hearing about Izuku's backstory and remembered that his Izuku had a similar quirk but he was unsure if they were the same. He went off to confront Izuku.

Flashback end

The first battle starts with Izuku verses Shinso and the latter easily wins despite Shinso making an effort to use some martial arts. Izuku battles using fire daggers btw.

Katsuki, Shoto, and Izuku being absolute beasts continue to plunge through the tournament until it was Shoto vs Izuku. Just before they went on the field, Endevour confronted both twins. He first went to Izuku and said that he an interesting quirk. Izuku was angry that he was here but held his anger and coldly told him, "If you want to confront something, confront your past and family". Endevour was shocked and confused as he didn't how much the greenette knew.

He then went to Shoto and weakly told him just to use his flames and be over it. Shoto was conflicted after having a talk with Izuku and simply told him to "fuck off" (Yes I did it XD).

The battle happened so quickly, Izuku started off with blasting his way across the field using his red flames. Shoto opted to use his ice (duh) and simply tried to keep him away. When Izuku got closer however, he shot off his red-black flames but for brief moment turned to light blue. The crowd, especially 1-A was shocked as it means he was simply sad.

Shoto asked why he was going so far as he barely used his quirk on the others. Izuku snapped that he was furious Shoto wasn't using his left side. Before he could use the same excuse again, Izuku gave out the most iconis line...

"IT'S YOUR POWER ISN'T IT!!? YOUR POWER NOT HIS!!"

Shoto stopped to process the information before almost being bulldozed by Izuku's literally flame body. Izuku went on to say that they were more similar in blood and finally revealed his identity to the whole crowd.

Izuku: Like I said, you and Enji are more foolish than I thought!!

Shoto: I-... How do you know about the old man!?

Izuku: More like our old man. Don't remember what happened before your 4th birthday?

Shoto: I- what- WHAT DO YOU KNOW!!?

Izuku: DON'T TELL ME YOU DON'T REMEMBER YOUR TWIN, SHOTO-NII!! THE PAST NEVER DIES!! (I'm crying rn)

Shoto: Ototo...

The whole stadium were astonished that the presumed dead Todoroki twin of Shoto was alive. Endevour was especially shocked and remembered the small talk he had with him. The Todoroki family especially were in tears watching the battle from their home. Inko merely sighed at Izuku's reveal and ignored the nurses questioning looks.

Class 1-A and Katsuki shouted outbursts about the reveal while also rooting for both twins to win. As Shoto finally used his fire, Izuku gave out a shit-eating grin and the fires on the battlefield kept fliking from yellow, to red, to light blue, something the class excitedly noticed. Izuku used both red and light blue fire so he could literally be on equal grounds with Shoto. In the end, Izuku won by becoming a human tornado (imagine spinjitzu) and blasting Shoto away using a Super Move: Tornado Twin Strike.

Both students were sent to the infirmary receiving multiple wounds from their quirks. Shoto had a simple case of heatstroke and frostbite but Izuku had extreme quirk exhaustion coupled with a headache from switching between emotions rapidly. Shoto tried to engage in a conversation with his twin but Izuku only tells him to think through his own heart instead of blood. 

The fights were paused for fixing the damage the twins did and the news that the missing Todoroki twin appearing spread like wildfire. Even Present Mic was confused whether to call Izuku Midoriya or Todoroki. Once the battlefield was fixed, they moved on to the next fights and it finally got to Katsuki vs Izuku. Katsuki however was angry that his rival's "ultimate battle" was with his lost twin. The fight ended shorter than anticipated (further angering a certain pomeranian).

The conclusion of the tournament was Izuku gaining first place, Katsuki in second (while being chained), and Shoto in third. Once the Festival was over 1-A bombarded Izuku and Shoto with their connection the former merely shooed them. He did however invited Shoto to visit Rei as he was going to visit Inko. He accepted and the two mothers bonded over their "dorky" boys.

During a break from UA, Shoto invited Izuku to meet his family again, aaaaand the ride there was awkward. Izuku warned him that he already knew about the abuse and warned him having Endevour in the same room might make it worse. He understood and texted his family. They were greeted by tear-blinded Fuyumi and Natsuo seeing their little brother back. They managed to catch up with everything they missed but left out Endevour.

On the day of the Internship, Izuku decided to pick Edgeshot. he answered the others questioning gazes that he doesn't really like the spotlight and he emphasized that unlike him, a certain someone didn't even practiced half of his quirk. Izuku preffered hand-to-hand fights instead of quirk fights as Heartburn is unpredictable. He picks his Hero Name as Chroma Flare from his vivid fire colors.

The public caught wind that Todoroki Twin (though Izuku insisted that he's still Midoriya) went to Edgeshot Agency instead of his father's.

He learns a lot from Edgeshot and during the Hosu incident saved Iida from the Hero Killer. Stain actually expressed interest in him since he was "the number one fake's" lost child. Izuku admitted that he dislikes Endevour and he doubt he can forgive him, but he also saw that he was willing to change and he'll accept that. Shoto soon joined the battle and all three of them amanged to defeat him.

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THATS IT!! I AM DONE!!!

This is just my imagination and all of you can do WHAT EVER THE HECK YOU WANT!!! This took all of the energy out of me.

This isn't the full version but I don't care right now. nd before you ask, Dabi saw the fight and is conflicted whether to come home or not.

Hope you enjoyed it!!