Izuku had been wondering about the vault door inside the Void ever since he'd first laid eyes on it. The door looked exactly like the one from his vision of the First One for All holder's past, except with more locks. (Wasn't that ominous?) From what Izuku could tell, the contents of the Void seemed created by the vestiges who inhabited it. But why would Yoichi want to duplicate his old prison? Why would any other vestige for that matter? During his training with the vestiges, Izuku had asked them about the door, but none of them had ever seen it. Second had looked at him funny for even asking. Izuku would swear that Yoichi had been sweating. Since when did ghosts sweat? Very fishy…
Not for the first time, Izuku found himself inside the Void during a dream. As if in response to his curiosity, the shadows pulled back to reveal a familiar metal surface with a dozen locks. The vault door hung suspended in space. Time to find out what was behind it. When Izuku stretched out his hand and willed the locks to vanish, the door swung open.
Instead of a metal bank vault, the other side held a child's bedroom. Action figurines spilled out of the russet wood cherry trunk and across the floor. The bookshelf had been double-stacked, but more books still covered the spiral rainbow carpet. Cartoon posters and childish doodles covered the tan walls. A huge superhero poster spread across the entire ceiling, with a cast of colorful characters and a looming demon king.
A young boy lay tucked into bed under a Spiderman blanket. With his straight white hair and green eyes, he looked like a younger version of Yoichi. An older white-haired boy sat in the chair next to him, with a book open in his lap. The older boy read, "Despite his heavy injuries, the hero stood up and charged at the demon king again. Hey, are you listening?"
The miniature Yoichi yawned. "I think…I'm about to fall asleep."
"Then I'll finish the story tomorrow." The older boy closed the book, then leaned over and kissed the younger one on the forehead. "Good night."
"No, no! Finish it now!" Young Yoichi clutched at the older boy's sleeve. "Please, big brother!"
Izuku gasped. Big brother? Then this was All for One? He should have guessed sooner. He'd never seen All for One's unmaimed face, but he looked a lot like Yoichi. But why would a young All for One be inside the Void?
The older boy did not react to Izuku's sound. But young Yoichi looked up. Green eyes fastened on Izuku, then narrowed. Yoichi snapped his fingers. All for One froze in place. Throwing off the cover, Yoichi stood up. He padded across the carpeted floor, hopping over toys.
As soon as Yoichi stepped out of the vault door, his Spiderman pajamas changed into his usual long sleeved white shirt and jeans. He grew taller and leaner until he towered over Izuku.
"What is going on?" Izuku demanded. "Is that a vestige of All for One?"
"Let's talk after I secure him." Yoichi closed the vault door on the frozen scene inside, then locked it a dozen times. When Yoichi turned to look at Izuku, his face developed weary lines. "As you guessed, that's a vestige of All for One, copied way back when he first held the stockpiling quirk."
"It makes sense. All for One forced the quirk on you. He's the zeroth holder," Izuku mumbled. He glanced up, gaze sharpening. "The other vestiges don't know, do they?"
"No." Yoichi looked away, ashamed. "They're happier not knowing."
Izuku shook his head. "If he's dangerous, then they have the right to know."
"He's harmless. I swear it. Any of them could defeat him here." Yoichi twisted his hands. "I found big brother's vestige in this place when I first died. I was stronger than him here, for once in our lives. Maybe it's because I'm a true ghost and not merely a quirk ghost, or maybe because this place came from a merging with my original quirk. Either way, I could control the mindscape and he couldn't. Nor could he use any of his quirks. I overpowered him and locked him up."
"Then why…?" Izuku waved his hand at the door. "Why the fake child's bedroom? Why can't he see me?"
Yoichi said, "This is the only way I can keep him happy. When I imprisoned him, he was miserable. So I let him return to a happy time in our childhood when he looked after me."
"But why would you want All for One to be happy?"
Hanging his head, Yoichi whispered, "He's still my brother."
From a moral perspective, Izuku doubted if All for One deserved this mercy. But at least the delusion kept the villain from trying to escape and cause more trouble. Perhaps such a pathetic situation suited All for One. "Why keep this secret from the others?"
"They all hate him. They'd never understand why I created a pleasant illusion for him, and…" Yoichi hesitated. "They might even hurt him. I couldn't blame them, after everything he's done to them. But he's still my big brother. Please, Nineth."
Izuku had never been able to resist a heartfelt plea. Besides, with his desire to save Tomura, he had no right to judge. Yoichi had supported Izuku back when he'd vowed to save his enemy. The situation seemed to be under control. Izuku said, "I won't tell. I promise."
Yoichi's shoulders relaxed. "Thank you."
"I still don't understand." Izuku shook his head. "How did you erase All for One's memories? I didn't think that was an ability that the Void gave you." It made Izuku a little nervous, frankly.
"I didn't," Yoichi said. "I faked an illusion where I pretended to let my older brother capture me, then de-age me. He thinks he's the one in charge, and I lost my memories. Sometimes I let him pretend to be ruling the world."
"That's…that would work, I suppose." Izuku gaped. "But why would you let him have that victory over you?"
"Because the only time big brother can be happy is when he has all the control."
"He'd never do the same for you. He'd never let you even pretend that you'd won."
Softly, Yoichi said, "I know."
Izuku didn't understand why Yoichi would still care so much about making All for One happy, even to the extent of acting like his brother's prisoner. But he decided to let Yoichi make his own choices. Perhaps Yoichi had created this strange situation for his own benefit as well, so he could spend time with the older brother who he clearly still loved. Yes, that seemed likely the more Izuku thought about it. The childhood fantasy was as much Yoichi's dream as his brother's. In the end, which brother was truly the one who couldn't let go? Who had entrapped who? Who was happy? Who had won and who had lost?
Glancing at the vault door, Izuku asked, "Uh, will he notice that you've left?"
Yoichi said, "I'm able to stop time from passing for my brother when I'm not present in the room. That's how I tricked him into thinking that he's imprisoned me. But I should go back and let him finish our bedtime routine."
"Right." Izuku cleared his throat. "I'll go." Izuku never, ever wanted to return to this creepy room again. He planned to block this from his memory.
Yoichi opened the vault door again and stepped inside, returning to let the vestige of his older brother finish his bedtime story.
Author's Note: Ironically, Yoichi just reversed what All for One did to him in "The Ultimate Vault." The only difference is that All for One would never, ever pretend that Yoichi won. He doesn't have that in him.
No matter what Yoichi pretends, his reasons are more selfish than he would admit even to himself. That's part of why he wants to keep this secret. The other vestiges might call him out on being a little too similar to his brother.
Thank you to MamaShenanigans for giving me the idea about what might be behind that vault door.