AN: One more question for today, so here it is!
Guest: Okay this is like... honestly one of my favorite questions I've been asked so far, because it actually made me think, for like a solid half hour, at work on a reason why Izuku wouldn't have already come up with this. Because like yes, it fixes basically his one problem, so why would he not be using it already by the time he got to UA? The answer I found was... honestly some science BS I pulled out, but the science DOES work so you should still think of it as legitimate reasoning. The reason he doesn't use it is because he tried once. Quite a few times. The first one he didn't think about breathability and started suffocating, but the next few worked fine, for about an hour, before he would start to get drowsy. It wouldn't make him pass out, but he would be running low on energy until he took it off. The reason behind this is because when he wears something skintight across his whole body, specifically his head, which he can't not cover if he's using something like this since that's the most vital part, it always causes issues with light. Besides food, Izuku's crystal body more commonly used light as a main energy source to run. When he has anything covering his head, the main spot where he absorbs light, then he starts to pass out. Even with a skintight covering that is transparent, because of how refraction works, that second layer actually causes more light to bounce off before entering his body, which leaves him with less energy overall. Like I said this is rooted in science but it's mostly just me trying to explain why Izuku, who is generally pretty smart, wouldn't already be using this.
As Izuku knocked on the door to the principal's office, he felt slightly jittery and unsure if his plan would work. He simply had to trust that it would, and go from there.
"Enter."
Izuku stepped into the room, seeing Nezu sit at his desk which looked comically large for someone of his stature to be behind it. The animal took a sip from his tea before responding to the boy's entrance.
Midoriya! To what do I owe the pleasure?"
"Well sir, I came to request that I be allowed to take something with me during the Sports Festival, specifically this," he said as he pulled his sword from its sheathe and lay it before the principal.
"Well, I would love to, but as you know we only allow Support Department and people who have required items for either safety or for their quirk to function at all to bring items with them. I know that your quirk doesn't require this sword to function, so why should I let you bring it with?" Izuku steeled himself before revealing the truth.
"Because Kuromu here actually a living blade that is encoded to my DNA, and is therefore a part of me." The principal leaned in at this, his interest piqued.
"Interesting, do tell me more."
"When I had Momo make this for me during the incident at USJ, I fused it with part of my body that held my cells," he explained, holding up the missing part of his right pinky, "And since then it became living, just as my crystal is living. I can generally sense what it means to say, even though it cannot speak on its own at this point. It does, however, have feelings, and does not wish to leave my general vicinity at any time."
"Well well... I believe that if something is alive and contains your DNA, that it would be best described as 'you,' wouldn't you agree? I'll allow you to bring it with, on one condition. You may not use the bladed end on anyone during the fighting, and if you do you will be immediately disqualified. I know how sharp obsidian is and we really don't need someone getting lacerated like that on live television."
"Understood, thank you sir." Well that went better than expected.
Two days later Aizawa announced that today they'd be doing joint training with the class of 1-B. They all got their gym uniforms on, since it would mostly just be sparring matches and they didn't need support gear for it, and head to an open area where 1-B was already waiting.
"Today we will be doing joint sparring matches with 1B," Aizawa called out, "Not all the matches will be specifically between 1-A and 1-B members but they will be one-on-one."
"MIDORIYA!" Monoma yelled, before Aizawa cut him off.
"The matchups will be decided at random, so no grudge matches. Every fifteen minutes we will reroll your opponents. Once we announce your first opponents, pair up and begin!"
Izuku found himself standing across from Kirishima, who seemed pumped up to fight against him.
"Hey, if you want me to I can keep the punches down, it wouldn't be manly of me to shatter you!"
Izuku grinned at the thought and decided he could afford to show off a bit. "I appreciate the sentiment, but I think it'd be far manlier if you came at me with everything you had! After all..."
Izuku threw his shirt off, revealing his chest, which was now almost entirely made of cloudy black crystal. After a few moments of concentration, the black spread outwards, covering all his limbs until he was made entirely of the material.
"I need to test out just how tough I am now!"