The impact rig took Momo forty-five minutes to build.
Yami had arrived at the training room to find her already inside, three precision instruments in various stages of completion on the table she'd claimed by the wall, the lipid shimmer at her forearm steady in the way it was steady when she was doing something well-practiced. She was consulting a handwritten diagram — she brought handwritten diagrams to things she'd been thinking about, which was something he'd learned in the library — and making adjustments to the pendulum's suspension mechanism with the focused attention of someone calibrating, not just building.
"The sensors are the complicated part," she said, without looking up. "I can create the mechanical components accurately, but the electronic calibration needs—" She paused. "Sit. I'll finish explaining when this is done."
He sat.
The rig was finished in the forty-fifth minute: a pendulum arm attached to a weighted cylinder, adjustable by counterweight to deliver calibrated impact forces, with sensors at the contact plate that Momo had apparently also created, which she explained were designed to measure absorption differential rather than raw force — the difference between impact delivered and impact transmitted to the target.
"The fragment," she said, which was the word she'd chosen for it. He hadn't corrected her.
"The fragment," he agreed.
The first twenty impacts were at the lowest calibration — light, consistent, the force level of a casual hit from someone without a strength-based quirk. The absorption differential held in the eighteen to twenty-four percent range across all twenty. Momo graphed it in real time on her tablet, the data points clustering in a band that confirmed what he'd measured alone but with three times the precision.
The next twenty were at moderate calibration. The band held for fifteen, then stuttered at sixteen — no absorption, full transmission — held for three more, stuttered at twenty. Momo noted the malfunction instances without commenting.
At the high calibration, the malfunction rate jumped.
She looked at the graph. Then at the heart rate monitor she'd also built, which was currently reading his BPM via the contact patch on his forearm. "One hundred forty-four when the last malfunction occurred," she said. "One-thirty-one for the one before that, but the data's within range of the others." She looked at the graph again. "Your heart rate is the variable."
"That's the pattern," he confirmed.
She made a note. "High-stress deployment is unreliable." She didn't ask what the fragment was or where it had come from or whether the mechanism she was measuring had anything to do with his registered quirk. She measured what was present and mapped it. He was developing a theory that this was simply how Momo Yaoyorozu operated: she would take the data she had access to and build a model, and she would not require more data than she had been offered, and she would build an accurate model from whatever was available and save the questions she still had for a moment when the questions were relevant.
"Above a hundred-forty BPM, the fragment's effective protection drops significantly," he said. "In any situation where the fragment is most needed—"
"It's least reliable." She closed the graph and set the tablet down. "Yes. That's the challenge." She looked at the rig she'd built, then at the wall. "We can address the threshold. Training toward lower baseline heart rate in high-intensity situations — it's not the same as fixing the malfunction, but it pushes the reliable range higher." A pause. "I can help with that too, if you want."
He did.
Tokoyami was reading when Yami approached him at the library's window table, which was Tokoyami's consistent location — the specific corner where Dark Shadow didn't have to contend with the ambient brightness of the room's main lighting and which had a sightline to the door that satisfied whatever threat-assessment instinct was native to either Tokoyami or his quirk. He looked up from the book with the specific expression of someone who had been expecting an approach and had been developing opinions about it.
"Sports Festival," Yami said.
"Cavalry battle," Tokoyami said, which confirmed both that he'd been thinking about it and that he'd already identified the relevant event. "You want a team."
"I want your team."
"Dark Shadow's team," Tokoyami corrected, mildly. "He has opinions."
"I've noticed." Yami pulled the chair across from him. "The cavalry battle favors defensive formations. Dark Shadow is an outstanding deterrent — any team that threatens the headband thinks about what intercept looks like before they commit. Momo's on my team, which means we have creation-based response for any variable the cavalry throws at us. I need someone with area-sensing capability to cover the approach vectors we can't watch directly."
Tokoyami considered this. Dark Shadow was a low-profile presence at his shoulder — darker than the ambient shadows, the sentient quality of it visible to anyone paying attention in the subtle way that a watching animal was visible even when still.
"You're asking for a team, not followers," Tokoyami said.
"Correct."
Dark Shadow extended toward him — not aggressively, more the way a cautious animal extended when checking something unfamiliar. Yami sat still. The sentient quirk got within two feet and stopped, and the stop had the quality of a reconsideration.
"He'll think about it," Tokoyami said.
"I'll take that."
Jiro was in the music room after class, which was where she was most days after class, the earphone jacks plugged into the room's acoustic system in a way that suggested she was checking something she'd been listening to rather than performing. She looked at him when he knocked on the open doorframe with the expression of someone assessing an interruption for its interruptive content before deciding.
"Cavalry battle," he said.
She unplugged one jack. "Who's on the team."
"Momo, Tokoyami, you."
She looked at him for a moment. "Tokoyami said yes?"
"He said he'd think about it. Dark Shadow seemed to be the deciding vote."
She pulled the second jack and wound it around her fingers in the automatic way of someone doing something while thinking about something else. "You've got Momo for creation and Tokoyami for area denial. What do you need me for?"
"Ground vibration sensing. You can detect approaching teams before we see them through earphone jack feedback." He paused. "Also: nobody's going to expect the quiet ones."
Her mouth did something that wasn't quite a smile. "That's actually a decent pitch." She rewound the jack once more. "You already had this team picked before you asked, didn't you."
It was not a question.
"Yes," he said.
"At least you didn't pretend otherwise." She gathered her bag. "Fine. I'm in. But I'm not doing anything that makes me look like dead weight on camera, so we're running actual drills."
"Wednesday afternoon, after school."
She confirmed with a single nod that had Aizawa-level economy.
The practice session ran for ninety minutes in the lower training field, the four of them working through cavalry battle positions with the focused energy of people who understood that the event's actual challenge was not strength but coordination. Momo produced practice materials at the session's edge — a test headband, a practice rider's harness — with the efficiency of someone who had already considered every supply need before arriving. Tokoyami confirmed Dark Shadow's outer-perimeter role with one demonstration that made both Jiro and Yami take a half-step back.
At the water break, the late afternoon light came through the training field's mesh walls at the angle it came through at that hour, cutting diagonal across the concrete, and Dark Shadow extended from Tokoyami's shoulder toward Yami's position in the same tentative way it had in the library.
It got closer this time. To within six inches.
Then it recoiled.
Not violently — more the way a hand withdrew from unexpected cold, the body's reaction to information that didn't match expectation. Tokoyami looked at it, then at Yami, with the expression of someone witnessing something that required annotation.
"He doesn't usually do that," Tokoyami said.
"What's he reacting to?"
Tokoyami was quiet for a moment in the way that meant he was receiving something. "He says your energy is wrong. He says—" Another pause. "He says it smells like something that should be dead."
The field held the after-practice quiet. Jiro looked between them. Momo's pen had stopped moving on her tablet.
Yami looked at Dark Shadow — at the sentient quirk hovering at the edge of contact with a person who had died three times and carried a piece of something that had absorbed shockwaves for an engineered biological weapon before being extracted and transferred. He thought about what category of wrong a being of shadow and instinct would use for the particular energy signature of a resurrection quirk with a B-tier fragment embedded in it.
"He's not wrong," Yami said.
He said it as lightly as it could be said, which was not very lightly, and picked up his water bottle and took a long drink, and Tokoyami watched him with an expression that had added several new items to its internal list.
Dark Shadow had never been wrong about danger.
He'd been carrying a piece of a weapon designed to kill All Might, embedded in his body like a second pulse, and a sentient quirk born from shadow had found it and named it with the accuracy of something that perceived things through a different channel than logic.
He'd file that.
The train home took forty minutes and he squeezed Momo's stress ball for thirty of them, watching the city move past the window, and the faculty meeting notes — which he didn't have access to and which he was constructing from available evidence — settled into something that resembled a picture he'd rather not have in full resolution.
Red tab. Three in the whole school.
He'd been in UA for six weeks.
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