The day after the battle with Explode, as Strengthen is at school, he overhears talk of an "invincible man", that's been going around fighting people outside school grounds, he thinks nothing of it, understanding it to be just someone who's really good at fighting.
Upon leaving school, he sees the ‘leader’ of bullies from before walking around shirtless, yelling at people leaving school to fight him. Upon seeing Strengthen he challenges him to a battle, swearing to have revenge for his humiliation from before. Strengthen ignores him and walks past.
Extremely angered, the bully grabs Strengthen's shoulder, and when he turns to look at the bully, the bully punches him in the face, knocking Strengthen down to the ground, a bit hurt. The bully begins gloating about how regardless of what fluke made Strengthen suddenly have a glow up like that, he wasn't going to let him get off scot-free.
Seeing as there was no option other than to fight him here, Strengthen gets up and quickly lunges at the bully, retributing by punching him in the face. Weirdly enough, nothing happens, and even though the punch connected, it seemed to have done no damage to the bully, who retaliates by punching Strengthen in the gut, before walking back a bit, saying:
"Y'know, I have no idea what happened but, when I was walkin' ta school today I suddenly got this weird power that makes me negate stuff that happens ta me. I dunno why it happened, but now I can serve yer ass for what ya did to me yesterday."
Recognizing that this is the same kind of power as his own, Strengthen starts trying to think of a strategy to beat Negate while running away, but no matter what he tries to throw at him or hit him with, every damage is negated and just ends up being useless. Interestingly, when chasing after Strengthen, Negate ends up tripping on the sidewalk, hitting his face on a light pole, which ended up hurting him.
Strengthen continues to run away, leading Negate into different areas to see and understand what the limitations of negation are. Eventually he reached the site of the battle against Explode, which since then has had security tape placed around it, but due to the event's recency no one has been assigned to work on repairing it yet.
Hiding among the rubble, Strengthen tries to hit Negate by surprise, but regardless of how fast he goes, Negate is able to notice him just fast enough to know the attack is coming, negating its effect. Cornered and now having revealed himself, Strengthen has his back against the wall, and is forced to use his power to become even stronger, so as to be able to hit the wall behind him, causing it to destabilize, making it so the loosely held roof above him and Negate to fall unto them.
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This works, and is able to hurt both Negate and Strengthen, although Strengthen less so. Neither are knocked out, but the effect it had on Negate is noticeable. As the both of them get up from the rubble, Negate comments on how Strengthen was able to suddenly grow in strength like that, and how that must be the reason he was able to defeat him yesterday after being so puny for so long. Negate says that Strengthen "cheated", and not a real man for it. Strengthen replies by saying:
"If I'm a cheater, then what are you?"
Negate is angered by this, and lunges at Strengthen. Strengthen kicks Negate's leg, somehow actually causing him to fall, and get hurt upon landing.
"But how?! I negated your kick!"
"I understand how your power works now. You negate an action by removing the effect that action had on you. Even though you negated the damage my kick would've caused on you, my leg stayed still, causing you to trip on it as if it was any other inanimate object. You can't negate things that you don't recognize as actions, can you? Especially if you don't notice that thing is happening to begin with until it's too late."
Seeing as his shtick has been revealed, Negate gets up and tries to punch Strengthen in the face, blinded by anger. Strengthen dodges the punch, and grabs Negate's arm over his shoulder, preparing to throw him. Of course, this is negated, but with the effect of being thrown negated, Negate falls straight down as Strengthen lets go of him.
Negate keeps getting up over and over again, but with Strengthen's stronger body and creativity, and Negate's lack thereof, Strengthen is quick to knock out Negate through roundabout methods, finishing him off by standing still when getting punched in the face by Negate, and hitting him back in the stomach with a well timed punch that Negate wasn't able to see.
Later, Negate wakes up on the floor of Strengthen's house, with a rope tied around him. Explode comments on how simple of a method this is compared to the one he used against hers, but Strengthen explains how Negate shouldn't be able to negate continuous processes, but only immediate ones, otherwise he would've just negated "losing" the fight as it happened.
Negate begins yelling at Strengthen, telling him to set him free and fight him like a man. Explode slaps him, which obviously does nothing, and only makes him yell even louder, but Strengthen tells Negate that because of that slap, he's a living bomb, and can explode at any moment, prompting Negate to shut up. In truth, Explode didn't even think of that, and just wanted to feel included by trying to make Negate shut up.
Strengthen explains what he knows about these powers, and tells Negate that he doesn't plan to do anything to him, but he needs to be careful using his powers like that in public, since no one knows what will happen if people find out about them. Negate reluctantly agrees, and is promptly set free from the ropes by Strengthen. He thinks of attacking him right now, but gives up knowing he'd lose. He walks away vowing to one day negate the humiliation he was forced to go through by Strengthen.
The next day the rumor going around at school is that Negate lost in a fight against Strengthen, making everyone laugh at him behind his back, so maybe it'll take a long time until he can truly "negate the humiliation" from it.