One day during Izuku's freetime, both Shoto and Kaine were helping him learn to control the fire half of his quirk.
"You need to learn to suppress your flames, with every attack you use to increase your strength, you create flames, if you can divide your quirk into two, you will have an easier time learning to control your power" Shoto explained.
Shoto did most of the actual training, while Kaine was writing in a journal with a pen using red ink.
Kaine walked up to Izuku as he hid the journal in a jacket pocket, "Izuku, take a deep breath, and try blowing fire out of your mouth at Shoto."
"What?!"
"If you have pyrokinesis as part of your quirk, then you should be able to breathe fire. Shoto can produce a wall of ice, and I feel like he is probably fire resistant, if not fireproof. Also breathing fire wouldn't be linked to any strength enhancing powers, so that will be the easiest way to learn to separate them."
"How do you know if I even can breathe fire?"
"Don't worry… I just do" Kaine had a strange smile and put a hand on where the journal was in his pocket.
Izuku closed his eyes and took a deep breath in through his nose. Holding his breath for a moment, he then breathed out of his mouth shooting a stream of fire at Shoto as he matched the fire with his ice powers.
"Good job Izuku! You aren't breaking any bones, you are actually controlling it."
Izuku stopped breathing fire and took a deep breath.
"My mouth feels like it's on fire…"
"Well it just was" Shoto said in a monotonous tone.
Kaine handed Izuku a bottle, and Izuku started chugging it.
"What is this?" Izuku asked, surprised the drink wasn't water.
"Aloe Vera juice, I bought a few bottles, in case anyone got burned. It will help your throat feel cooler. If you have pyrokinesis, you should be fire resistant, so your mouth shouldn't get blistered or burned. Just warm feeling."
"Thank you Kaine" Izuku said in between drinks of the juice.
"Now that you have a way to use your flames separately, you can learn to control them, and then learn to suppress them when you want to use your strength enhancing half of your power" Shoto mentioned.
"Hey Todoroki, you have a fire quirk also, why don't I ever see you using it in combat?" Izuku asked.
"I- I don't use my fire half during a fight, it's a promise I made to myself" Todoroki explained, refusing to speak anymore about it after that point.
They kept practicing Izuku's fire breath after that, as well as Izuku attempting to produce flames from his hands without using One for All.
...
Each night Izuku meditated in front of a candle, lighting it and extinguishing it with his quirk. Attempting to gain more control over his quirk.
But Izuku didn't set up any safety measures at first. Meditating in front of the candle in his room, he used his breath to ignite the candle with black flames. Then he put his hands near the candle, moving the flame side to side, lifting it off of the candle wick, suspending it in the air between his hands.
Then he sneezed.
The flame grew massively and caught his drapes on fire, "GAHHH!"
Midoriya started patting the flames with his hands attempting the put the fire out, rather than using his pyrokinesis to deal with it.
Luckily down the hall, Fumikage heard Izuku panicking and used his dark shadow to suppress the flames and extinguish them.
"Oh… thank you Tokoyami… that Dark Shadow of yours seems very useful."
"It's weak in light, I'm just glad your fire in black flames instead of bright ones that would weaken me."
"Oh, it's the opposite of that upperclassman, Kaine's quirk, he gets stronger in the light."
"Well I hope he and I never have to fight, as the two of us would cancel each other's powers out."
"DEKU YOU BASTARD, WHAT THE HELL ARE YOU DOING DOWN HERE!" Bakugo yelled out after smelling the smoke.
"I too would like to know why you are practicing such a destructive quirk indoors at this time of night." Ida called out.
Izuku's room was filled with each of the guys concerned about the smoke that had smelt. Izuku was mostly embarrassed about how his room was full of All Might memorabilia rather than about catching his drapes on fire.
If people have suggestions about how I should change the manga's story later on, with Deku having pyrokinesis, send me a private message. I want to do more original scenes, but I also know that scenes for the manga will become altered from Deku's new power. I mean I am working on the Attack on USJ and want to make that a little bit longer than what I've been doing so far, and I already found a few parts where the flames can make a difference. So yeah, any suggestions would be great, like for the tournament arc, or how he could implement pyrokinesis with one for all full cowl.