Reborn as Eren Yeager: The Gacha System

It started, as most disasters in Eren’s life seemed to, with Armin having a terrible idea.

"We should build a treehouse."

"Why."

"Why not." Armin said this with the total conviction of a boy who had clearly been planning this for several days and was only now unveiling it. "It’ll be a base. A headquarters."

"For what?"

"Things. Important things. Headquarters things."

"You have no idea what it’s for."

"I have some idea."

Mikasa, sitting cross-legged in the grass sharpening a stick into a point with far more focus than the task deserved, didn’t even look up. "He wants somewhere to read where his grandfather won’t find him and confiscate the book."

Armin went beet red. "That is not — okay, it’s partially that."

"It’s entirely that."

"It’s mostly that."

The treehouse-slash-headquarters took shape in the old oak behind the Yeager house over the better part of an afternoon, which was a generous way of describing what actually happened, which was three children with no carpentry experience attempting to nail planks to a tree while arguing constantly about the correct approach.

"That board’s crooked."

"All the boards are crooked, Armin, that’s the aesthetic."

"Treehouses don’t have an aesthetic, they have structural integrity, which this currently lacks—"

"Hold this."

"That is not how nails work—"

Mikasa, predictably, was the only one making actual progress, calmly hammering a plank into place while Eren and Armin conducted what amounted to a shouting-based architecture seminar six feet below her.

"How is yours already straight," Eren demanded.

"I measured first."

"That feels like cheating."

"It’s called planning, Eren." She didn’t even glance down, just kept hammering with the same unbothered rhythm. "You two argued for ten minutes about whether the door should face the house or the field."

"That was an important debate!"

"You decided nothing. I put it facing the field while you were still talking."

Eren looked up. She had, in fact, already finished an entire wall.

"...You’re terrifying," he said, with real admiration.

"Thank you."

By late afternoon the treehouse existed in a state best described as structurally optimistic — walls that mostly stood, a floor that only creaked in one specific, avoidable spot, and a door, facing the field, that didn’t quite close all the way but which Armin insisted "added character."

They celebrated by cramming all three of them inside at once, which the treehouse had not been designed for and expressed its displeasure by groaning ominously the entire time.

"Is that supposed to happen," Eren asked.

"Probably."

"That’s not reassuring."

"I said probably, not definitely."

Armin, wedged into the corner with his knees against his chest, produced a slightly crushed book from inside his shirt with the triumphant air of a magician revealing a card trick. "Headquarters business."

"You brought the book into the treehouse we just built — "

"That was the entire point of the treehouse!"

"You told your grandfather you were going to help build a fort!"

"I did help build a fort. I nailed at least four boards."

"You nailed one board and then supervised."

"Supervising is important work."

Mikasa, from her spot by the crooked door, finally allowed herself a small huff of laughter. "You two have been friends for how long?"

"Too long," Eren and Armin said, in perfect unison, then looked at each other in mild betrayal at having agreed on something.

The sun had started going gold through the gaps in the walls — gaps that were, Eren privately admitted, probably not intentional — when Armin finally packed up his book and announced he needed to get home before his grandfather noticed both his absence and the book’s.

"Don’t tell him about the treehouse," Armin said, halfway down the rope ladder that Mikasa had, of course, tied more securely than anything else about the structure.

"Why not? It’s a triumph of teamwork."

"It’s a fire hazard, he’ll make me tear it down."

"It’s not a fire hazard."

"There is an unattended stack of dry kindling six feet from a structure held together by optimism, Eren, it is absolutely a fire hazard."

"...Fair."

Armin disappeared toward home, leaving Eren and Mikasa alone in the fading light, the treehouse creaking companionably around them like it was settling into its new role as a load-bearing secret.

"You’re good at this," Eren said eventually, running a hand along the wall she’d built. Straight nails. Even spacing. It put his and Armin’s side to visible shame.

"Good at what?"

"Building things. Measuring things. Being competent in general while Armin and I make noise at each other."

"Someone has to balance you two out."

"You could’ve just let us fail. Would’ve been funnier."

"I considered it." The corner of her mouth twitched. "I decided against it at the last minute."

"Very generous of you."

"I try."

He sat down against the wall she’d built, testing whether it would hold — it did, without complaint, unlike his own section, which had already developed a faint but concerning lean.

"Can I ask you something?"

"You’re going to anyway."

"That’s not a no."

She settled beside him, close enough that their shoulders brushed, and didn’t tell him to move over. "Ask."

"Why do you keep making me things? The bird. The wristband. Half the thread in Wall Maria, probably, at this point."

She was quiet a moment, turning the question over the way she turned over everything — carefully, without rushing toward an answer just to fill silence.

"Because you keep things," she said finally. "Other people lose gifts, or forget them, or don’t notice when something’s gone. You don’t. You still have the first ribbon I ever gave you, and that was two years ago."

"You noticed that?"

"I notice everything you do."

It wasn’t said dramatically. It was said like a fact, plain and certain, the same tone she’d use to note that the sky was cloudy or the soup needed salt. Which somehow made it land harder than if she’d tried to make it sound important.

"That’s either very sweet," Eren said, "or slightly alarming."

"Both."

"I can live with both."

The light kept fading, gold sliding toward orange, and for a while neither of them said anything else, content to sit in a badly-built treehouse that was, against all reasonable expectation, still standing.

That night, alone in the warehouse, Eren allowed himself the daily draw before he let himself think about anything else.

[Daily draw available. Spinning...]

The wheel spun, slowed, stopped on a category he hadn’t seen roll up before.

[Result: Minor Precognitive Flicker (Tier 2). Occasional flashes of imminent danger — seconds of warning, not detail. Passive. Uncontrollable trigger.]

Eren sat with that a long moment, turning the words over. Seconds of warning. Not a vision of the wall breaking. Not a map of what was coming. Just — seconds, sometime, somewhere, with no way to call it up on purpose. A single card pulled from a deck he wasn’t allowed to shuffle.

Still. Seconds could be a lot, if they landed at the right moment.

He thought of Mikasa’s hand catching his sleeve in the wind. Of the wristband, crooked but sure, tied by careful fingers. Of a treehouse leaning slightly on his side and standing perfectly straight on hers.

Seconds, he thought. I’ll take it. I’ll take anything.

One hundred and seventy-two days.

He closed his eyes and let the number sit there, quiet, counting itself down whether he was ready or not.