In Build's AR World, Tsutsumi, disguised in Bakugo's appearance, strolled casually into a dimly lit research room. Katsuragi sat hunched over his computer, fingers dancing over the keyboard.
"How ya doing?" Tsutsumi asked lightly, setting a heavy bag on the table with a dull thud.
Katsuragi didn't look up at first. "Working on an upgrade for my Rider System." Then his gaze fell to the bag. "What's this?"
"A few Full Bottles you might need."
Katsuragi's eyes widened. He unzipped the bag, his expression twisting into disbelief as he stared at every single Full Bottle from Touto.
"How did you-"
"It doesn't matter," Tsutsumi cut in, his voice calm but probing. "Now… tell me the real reason you created the Rider System."
Something felt off. In his memories, the man who wore Build's armor was a narcissistic, egotistic, yet fundamentally righteous hero. But this Katsuragi radiated none of that.
"The reason I created Project Build is-"
"Cial."
The voice came from the doorway. Tsutsumi turned to see Evol leaning casually against the frame.
"So, you're still alive," Tsutsumi said, standing and readying himself.
Katsuragi's hand flew to his belt.
Rabbit! Tank! Best Match!
Two glowing components slammed together, forming the familiar red-and-blue armor.
Hagane no moonsault! RabbitTank! Yay!
"-to take over the world," Build finished coldly, pressing his blade to Tsutsumi's neck.
"You?" Tsutsumi's eyes narrowed. This wasn't the same kind of man as in his world. "Isn't Project Build supposed to be for the betterment of humanity?"
"Better humanity?" Evol stepped forward, his gun now aimed at Tsutsumi's head. "Taking over humanity sounds better, don't you think?"
Then it hit him, the one who placed the belt on to protect people's tomorrows, futures, and hopes. The one who fought because he wanted to help people. Who rose up to protect people.
That world... Katsuragi Takumi wouldn't have done that... but Kiryu Sento.
This world, Katsuragi... hasn't become the narcissistic, egotistic, and righteous superhero... but he is a person who isn't afraid to harm others for his own benefit.
"I see," Tsutsumi said, locking eyes with Evol. "Guess I was wrong… right, Evol?"
Evol froze. Tsutsumi had just outed him without a single slip from his own mouth. Snarling, Evol pulled the trigger.
Attack Ride: Zone
Tsutsumi's body blurred. The energy shots tore past his head and struck Build instead, making him flinch.
"Most Kamen Riders have their own motives and destinies," Tsutsumi said, his voice cutting like steel. "But one thing they share… is that they wear the mask to protect, not to harm."
His gaze bored into Katsuragi's visor. "If you can't understand that… you don't deserve to be called a Kamen Rider."
"Who the hell are you to decide what I am!?" Katsuragi shouted.
"Just a passing-through Kamen Rider," Tsutsumi replied, turning his back. "Remember that."
Pulling out a card depicting a red-and-blue armored warrior, Tsutsumi slid it into his belt.
"Henshin!"
Kamen Ride: Build
Two armor pieces, one crimson, one cobalt, shot toward him from opposite sides, locking into place with a metallic snap.
Hagane no moonsault! RabbitTank! Yay!
Katsuragi's eyes widened. "What? How did you?" He froze at the sight of the same armor on the boy they had planned to ambush.
It wasn't a perfect match. Tsutsumi's suit only carried the outer pieces, leggings, chestplate, gauntlets, and the mask, without the usual inner armor layer.
So this is what I look like without fully accepting Decade's power,
Tsutsumi thought, glancing down. The missing inner layer offered less protection, but with his hero costume already black, the gap wasn't as noticeable.
He locked eyes with the other Build and lunged, summoning the same weapon and swinging hard. The blow sent Katsuragi crashing into his computer.
Seeing that Katsuragi wasn't much of a fighter, Tsutsumi shifted his focus to Evol. Their weapons clashed in rapid exchanges before Evol disengaged, switching to long-range fire to keep him at bay.
Tsutsumi backflipped out of the way and slid another card into the belt.
Final Form Ride: Build
His punch connected with the other Build, whose body glowed, then split into a mechanical red rabbit and a blue tank.
"W-Wait, what is happening?!" Katsuragi shouted, bewildered.
"Attack!" Tsutsumi commanded.
The red rabbit darted around Evol, striking from every angle, while the blue tank rolled into position and opened fire in a relentless barrage.
"This isn't good," Evol muttered, retreating.
Final Attack Ride: B-B-B-Build
A white energy chart shot upward, wrapping around Evol and pinning him in place. The rabbit and tank charged down the glowing path, slamming into him just before Tsutsumi's kick connected, triggering a fiery explosion that sent him tumbling.
As the smoke cleared, Evol's armor flickered away, revealing the bloodied "shady uncle" beneath. Before Tsutsumi could finish him, Evol fired a stream gun, releasing thick black smoke and vanishing into it.
"Tch." Tsutsumi clicked his tongue, turning his gaze toward the red rabbit and blue tank.
Final Attack Ride: De-De-De-Decade
The rabbit and tank zipped back together, returning back into the other Build, merging just in time for Tsutsumi's finishing kick. The impact detonated, obliterating the lab in a thunderous blast.
When the dust settled, Tsutsumi walked out calmly, Build's card set in hand, his bag slung loosely over his shoulder.
...
After what felt like an eternity of running, Tsutsumi finally returned to his world, hands weighed down by the full bottles he'd managed to gather before the label of public enemy number one forced him to vanish from sight. His steps were heavy, each one echoing the cost of what he'd done.
He let out a tight, weary sigh and headed straight for a cold bath, hoping the chill might wash away more than just the sweat on his skin.
His memories stirred like murky water, reminding him of the ugly truth. Riders who twisted their powers for selfish ends were branded Dark Riders. How poetic, then, that one of the first Rider worlds he crossed into was actually one.
And yet… wasn't he no different? Decade, after all, was the name once carried by Dai Shocker's own leader.
His breath escaped in a slow exhale. "What I did… was it really the right thing?"
The thought dug in deep. Who was he to stand in judgment over others, when the very roots of his power were steeped in the same evil that had plagued Kamen Riders for generations?
"Do I even have the right… to be judge and executioner across these worlds?" He leaned back against the icy wall, the chill biting into his spine, and pulled out his Decade card.
A dry chuckle slipped from his lips. "Funny… the more I learn about what it means to be a hero, the more unworthy I feel."
He slid the card back into his holder, sank lower into the cold water, and let the silence seep into him, numbing thought and feeling alike.
When he finally finished his bath, exhaustion clung to him like a second skin. He crawled into his futon, seeking rest.
The window burst open, the night air rushing in, and before he could react, Jiro was already dragging him toward school.
...
"Don't tell me you stayed up all night doing that thing boys do when they're alone," she teased, clearly fishing for a reaction.
"You mean playing a game? I don't even have a mobile game on my phone." Tsutsumi replied flatly, stifling a yawn.
"No, I mean the kind of thing you do in bed with your hand," she pressed, her smirk widening.
"I see… so you've been into that sort of stuff. I'll need to have a talk with Mr-"
He didn't get to finish. Jiro's foot slammed into his side.
"Oi! I was joking! No need to get me grounded for it!" she snapped, irritation in her voice. Honestly, his ability to crawl under someone's skin was borderline supernatural.
They continued toward school, Tsutsumi's mind wandering back to last night's events in the other world. He hadn't gained Build's power by defeating or befriending him, rather, he'd grasped the idea and core philosophy Build was founded upon.
By truly understanding the heart of Kamen Rider Build, it felt as though he had earned Build's trust, sharing the same desire to fight for a world of love and peace.
As they approached U.A., Tsutsumi's gaze landed on a wall of reporters crowding the school gate. Apparently, word had leaked that All Might was teaching here, and now the media was desperate for confirmation, or better yet, an interview.
"Tch… how are we supposed to get in like this?" Jiro muttered, eyeing the mob.
"Simple. Follow my lead, act natural, and don't answer anything." Tsutsumi said with quiet confidence, taking her hand and walking forward.
"Hey, you! How are All Might's classes?" a female reporter asked, shoving a microphone in their faces. Others quickly joined in, cameras flashing.
From the corner of his eye, Tsutsumi spotted a few classmates caught in the chaos, but since he didn't know them, he kept walking, ignoring everything.
"What do you mean by 'All Might's classes'? Is he teaching here?" Tsutsumi asked, face blank, tone genuinely puzzled.
The reporters hesitated, sensing he was clueless, and turned to Jiro.
"She's my classmate," Tsutsumi added before they could speak.
That was enough, their interest died instantly, and they moved on to pester someone else.
"Where did you even learn to lie with a straight face like that?" Jiro asked, baffled. Her childhood friend was not exactly known for social grace.
"I got bored and decided to learn," he replied casually. In truth, there was a world where his identity had been that of a phantom thief, so lying convincingly was simply a survival skill.
...
Inside Class 1-A's homeroom, Aizawa's eyes swept over the room before settling on the injured Tsutsumi and Bakugo.
"What happened?" he asked, voice flat but gaze sharp.
"We fell down some stairs," Tsutsumi replied without hesitation.
The room went quiet.
Everyone: "..."
Aizawa's eyes narrowed. Yeah, no. He clearly wasn't buying it. His attention shifted to Bakugo, whose injuries were even worse than Tsutsumi's. With Bakugo's personality, there was no way he'd cover for someone who beat him up.
"You heard him," Bakugo muttered, irritation lacing his tone. "We fell down some stairs."
Everyone: "..."
Nobody believed them. But since both had stuck to the same story, there wasn't much anyone could do.
Aizawa exhaled a tired sigh. "Forget it." He straightened, his gaze sliding to the rest of the class. "Good work in yesterday's combat training. I saw the video, and the results."
His eyes briefly landed on Tsutsumi. "Your view on heroism… not bad."
Not everyone could be like All Might, leaping across cities and throwing punches strong enough to change the weather.
"Now," Aizawa continued, "let's get to homeroom business. Sorry for the late notice, but today you'll be…"
The class collectively tensed.
"…deciding on a class representative."
A wave of relieved sighs swept through the room, finally, something normal.
"I want to do it!"
"Ooh! Ooh! I wanna be the class president!"
"Oh, you mean my position?"
"I'll do it! Pick me!"
Tsutsumi glanced around, baffled at the sudden enthusiasm. Normally, the position of class president was a headache no one wanted. But in the hero course, it carried weight, whoever held it naturally became the leader.
Still, to these students, it was less about leadership and more about bragging rights.
His eyes drifted to Midoriya, who was raising his hand eagerly.
Last night, Bakugo had wanted to beat Midoriya into the floor, but they'd agreed not to cause trouble, it'd risk exposing their… extracurricular quirk use. That didn't mean they were suddenly on friendly terms, though.
If Midoriya got in their way like that again, both of them would happily team up to crush him.
If you're going to stick your nose in other people's business, you'd better be ready to have it broken.
The class erupted into a heated argument over how to choose the new class president. Meanwhile, Tsutsumi, slouched back in his seat, mimicking Aizawa, and drifted off to sleep.
In his dream, an endless field stretched before him. Corpses and shattered armor lay scattered, fallen Riders, massive beasts, and nameless monsters alike. The air reeked of death and smoke.
And in the very center of this devastation… he stood alone.
The weight of it pressed into his chest. His eyes snapped open, a single tear slipping down his cheek. He wiped it away before anyone could notice.
By then, Aizawa had just finished talking to Ida and was now walking toward him."Congratulations," Aizawa said casually. "You're the new class representative. Try to finish your duties before sleeping in class next time." Without another word, he walked off.
Tsutsumi: "..."
Tsutsumi stared at the board.
"…How the hell did I get nine votes? That's half the class."
Since he hadn't even participated in the discussion, that made it even stranger.
"Your view of heroism is really inspiring," the frog-faced girl said sincerely.
"Yeah, and you're really strong," Momo added with a polite smile.
"I believe your darkness can also be a guiding light," the bird-headed boy said seriously.
"The way you moved during the combat trial was amazing. Let's spar sometime!" the boy with the large tail chimed in.
"Tch." Clicking his tongue, Tsutsumi leaned back, propped a foot on his desk, and barked, "Glasses, go buy me lunch."
Ida: "..."
Everyone: "..."
"W-What?!"
The rest of the class stared.
"What? Can't follow a simple order?" Tsutsumi asked with a mocking gaze.
"Tsutsumi-kun! That's bullying, you can't abu-"
"Forget it," Tsutsumi cut him off before he could finish. "You pride yourself on speed, yet you're this slow to follow orders? Waste."
Pushing himself up from his seat, he headed for the door. "Vice president, whoever you are, watch the class while I'm gone."
And just like that, he was gone without a trace.
The class sat in silence.
"…Maybe picking him was a mistake."