It goes like this:
Mirio is walking down the street a few blocks from Sir's agency, doing his patrols as normal, when he sees Overhaul himself wandering down the street. A pair of his direct subordinates walk beside him, judging from the plague masks on their faces, and the killing intent rolling off of them has every civilian avoiding them by a full block. It doesn't help that they're walking in the middle of the street, and the pair are slowly wrecking any car that comes too close. Barriers protect them, and the big one's punch destroys any vehicle that gets too close to Overhaul's path.
Before he can step in or call Sir, there's a civilian walking into the street out of nowhere looking determined to do something. Mirio really doesn't like it, but Overhaul stops a decent way away and stares at the boy when he doesn't move out of their path.
"Are you a hero? Coming to do the job even off duty?" He calls. Mirio chews his lip, praying that Sir will pick up because this looks really, really bad.
"You're the one who hurt her, aren't you?" The civilian asks, fists clenched so tight he's shaking. Emotions roll across his face in waves: fear, nerves, anger, desperation, determination. When Sir sends his phone to voicemail and sends a text, Mirio shoves his phone in his pocket and gets ready to rush in. Overhaul is already talking.
"Oh? So you're the one who took her from me. Give her back. She's necessary for my goals." He calls. On either side, the henchmen shift and get ready for a slaughter.
"I won't let you hurt anyone else." Civilian calls out, forming a rough fighting stance. Mirio screams internally because this kid obviously hasn't fought anyone before and he's already phasing through a parked vehicle when the big guy with fists of steel rushes out to attack.
Mirio is popping out of the ground in a rush when the stranger takes the punch against his arms. He phases through the attack coming his way and pauses on the other end of the street, eyes locked on the civilian. The pavement beneath his feet is cracked, nearly shattered from the force of the blow, and he's pushed back a full meter, but otherwise he's totally fine.
Everyone seems just as surprised that he made it out as Mirio is, which… he tries not to think about that.
"You took a punch, huh? Then I'll just go all out!" The thug cackles, rushing in for another blow. Mirio waits, watching as the civilian reforms his stance and makes a slight move to block, this time with one arm.
The pair dig a long track through the pavement as the Yakuza continues throwing blows. Each hit has the civilian slowly but surely pushed back, dug nearly past his knees into the street from the force of the onslaught. Mirio is shocked when the civilian is still standing after such an insane number of punches, but when the dust clears and reveals the pair staring one another down, Mirio finds himself shocked for a different reason.
Despite blocking most of the punches and taking a few to the chest, the stranger is just a bit dusty. Red like he's running a marathon, but not red from bleeding and bruising. His skin is flushed, not broken, and the muscle beneath is substantial enough that Mirio thinks he must be a hero-in-training, maybe Fatgum's nephew or something?
"Who the hell are you kid? You got a good quirk, that's for damn sure! No one ever goes two rounds with me!" The Yakuza cackles again, pointing a finger at the boy, "My name's Rappa Kendo! Call me Kendo, if you survive this!"
"…Deku." The boy states quietly, throwing off the shreds of his shirt and jacket. Mirio frowns as he runs a hand over his body, almost… appraising himself(?). "I guess that car thing wasn't just a fluke… Whatever. If it keeps me going, I'm not going to complain."
Deku settles into a fighting stance, looking more sure of himself now.
"I guess you're not going to let me take on your boss before you're down and out, are you?" He asks, offering a shaky smile. Rappa laughs even louder.
"HELL NO! I haven't had this much fun in too long! Let's go, round two!"
The shockwave is definitely unexpected. Rappa freezes, fist locked tight against Deku's, staring at the boy in disbelief.
"You…! Now you're gonna fight back, huh?! JUST HOW I LIKE IT!"
Compared to the first 'round', Mirio's eyes can actually track the fight now that 'Deku' is fighting back. Wind tears up and down the street in bursts every time their fists collide. Deku isn't being pushed back anymore.
When Rappa speeds up, Deku meets him. Mirio can only stare when their fists begin to blur into streaks of black and white. The street dissolves into a hurricane of wind around the pair. Mirio watches as heroes arrive.
Every single Hero stops on the edge of the conflict and stares, waiting because in the end, there's nothing else they can do. Moving against Overhaul is impossible with the random element and the barrier user.
A thunderclap breaks across the street as Rappa goes down beneath a right hook from Deku. Everything near the end had been beyond Mirio's eyes, which almost never happens. Only heroes like All Might and Edgeshot move at speeds like that, as far as he knows. When Rappa gets up and wipes the blood from his face, he begins to laugh again.
"Man… what… are you even… made of… Quirk like that… and you're just a brat…" Rappa barely steadies himself on his right leg, his form completely shattered, when he freezes entirely and glares at Deku. Mirio imagines he missed something, drinking in the injuries on Overhaul's man. "Don't lie like that, dumbass. This is a fight, not a FUCKING GAME! THIS IS A BATTLE OF MEN!"
"Lemillion." Sir's voice snaps Mirio from his staring. He hadn't even heard his mentor approaching which is… well, he's not proud of himself.
"Sir! Um, report… That kid, he came out of nowhere. It seems like he has someone Overhaul is looking for, which may have started the rampage. That person seems to be one of Overhaul's personal guards. I don't think that the boy, he calls himself Deku, is hurt. But he's moving…"
"I saw." Sir presses his glasses up his nose, fingering his seals with his other hand and glaring between where Overhaul is watching the fight and where Rappa is still screaming at Deku. "What set the Yakuza off like that? Is… Deku provoking them?"
"No, sir. He's been very quiet. Just… he's been getting better over the course of the fight." Mirio mutters, looking at Rappa's strangled screaming and a look of… shame? Resignation? He has no idea what Deku is feeling. Something is off but whatever it is, it seems to be between them.
"We can't go after Overhaul like this… I was hoping to save it, but I suppose there's no choice." Sir is already half covering his face, trying to hide the effects of using his quirk, when Rappa lunges forward. Mirio is lucky he had just turned back to the fight, but he still isn't sure he was fast enough.