Third person pov
Katsuki isn't sure whether or not he's crazy.
He thinks he must be. When he hears those whispers and can't find the source, it seems obvious that he's lost it. He wears Izuku's death like a heavy cloak over his shoulders. Not only is he incapable of letting it go, he's not ready to in general.
Even if he had the choice to move on, doing so would mean leaving Izuku behind again. Katsuki isn't strong enough for that.
So, he's going gradually more and more insane. Or he assumes as such, until he remembers the look on that blonde man's face. The widening of his eyes, the way he'd dropped his papers like they'd burned him. Katsuki's words-- Izuku's-- meant something to him.
But how? Katsuki would've remembered ever meeting a guy who looked like a walking skeleton. Would've remembered if he learned anything significant enough to the guy to make him shake in such a way.
Thinking about the alternatives makes him agitated. If Katsuki isn't hallucinating, then... then what? It's undeniably Izuku. Katsuki and Izuku grew up together, learning to crawl, to talk, to walk with one another. The longstanding friendship between their mothers had insured they spent maximum time together as children.
Katsuki would know him and his voice anywhere. He's impossible to forget, even more so now. It's practically ingrained into Katsuki's very blood.
The implications leave him nauseous and pensive. Even in death, what if Izuku can't find peace? Can't move on from the world that wronged him so surely? The thought of him drifting around, mostly unseen and unheard... it's tragic.
It made Katsuki want to scream and throw things. It ignited his smothered rage back full force, leaving him to agonize over the sliver of a chance that his childhood friend really was still trapped here.
He almost can't stomach it. The thought haunts him nearly as much as Izuku's actual death does. Izuku, faded into the background even in death. Left behind. Katsuki resists the urge to rip his hair out, forcing himself to breathe instead.
He tells himself that he'll sit down, gather himself up, and think about it carefully later. Right now he needs to focus. The trip to the USJ had the entire class excited, all of his peers eager to get out on the field and run through some rescue drills. Katsuki had felt relatively numb to it-- still did even now, only vaguely noting the architectural brilliance of the gargantuan glass dome when they stepped in.
The entire facility is split into different zones, the center plaza home to a giant fountain, the centerpiece to what is already a marvel of a building. Everyone oohs and awes, excited over the guest appearance of the Space Hero, Thirteen. Katsuki lingers near the back of the crowd in a daze, another night of no sleep weighing his limbs down like waterlogged clothes.
He can feel Mr. Aizawa glance at him every so often, as well as his redheaded classmate, but neither approach or say anything. If they had gotten the chance, maybe they would've.
It's funny how fast things can go sideways. Why the villains have decided to attack a class of first years, Katsuki doesn't know. Why do villains do anything? He doesn't know why he bullied Izuku, why he pushed him away with so much vitriol. If he'd wanted to be left alone, there were better ways to tell him to go away. Gentler ways, nicer ways.
But Katsuki's not nice. He's never been nice, and he probably never will be either. He wonders how it was so effortless for Izuku. He exuded gentle goodwill, sympathetic even to those who didn't deserve it, soft where needed, firm when he had to be. He was kind.

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What He Whispers (Ghost Deku)
FanfictionIzuku might've died, but he's not gone. It takes Katsuki a long time to realize this. (Or: A ghost Deku two-shot)