Zenitsu's mistake is thinking he's escaped just because he made it to the genkan without crossing paths with Kaigaku.
The punishment for his carelessness is being pinned to the front door, hot breath hitting the back of his neck as Kaigaku laughs, big hands undoing his belt and fly so Kaigaku can slip his pants down past his ass.
"Kai--Gramps--" Zenitsu calls, whether for help or to warn Kaigaku of how easily they could be discovered, he's not sure.
Kaigaku chuckles, licks up the shell of his ear. "He went to the dojo early today, little bro."
Kaigaku gleefully shoves three fingers into Zenitsu's mouth, thrusting them roughly as far back as he can get them. It’s done more for the joy of making him gag than for anything practical, but Zenitsu does his best to wet them with spit for his own sake.
"Let's see if you're still loose from last night, hm?" Kaigaku says as he pulls his fingers back out, brings them down to Zenitsu's exposed ass. "I mean, after four rounds even someone as uptight as you would have to relax a little."
Zenitsu's brain stalls on the number. Four? Four times?
"It wasn't--" he says, "It was only--"
"I fucked you four times," Kaigaku informs him cheerily, shoving all three fingers into him. It's uncomfortable, but well--Kaigaku's right, he's still a bit loose. It could be worse. "You think I care if you're awake or not?"
It feels stupid, to be so horrified by that, to cry harder because of that--Kaigaku's done all sorts of awful things to him, at this point shouldn't he be all cried out? But no, he never is.
"I think I liked it better, even," Kaigaku goes on, roughly pumping his fingers in and out of Zenitsu's hole, still sensitive from the abuse of the night before. "Fucking your ragdoll body. I Didn't have to hear your wailing, but you still cry in your sleep. Best of both worlds."
"W-what is wrong with you?" Zenitsu sobs out, trying and failing not to tense up when Kaigaku pulls his fingers out. He flinches at the sound of Kaigaku spitting into his own hand, the slick sound of him covering his cock with it filling his ears.
"What’s wrong with me ?" Kaigaku asks, and there's an edge of manic rage in his voice as he lines up, shoves his cock into Zenitsu’s poorly prepared body without ceremony. Zenitsu bites down on his lip to avoid screaming. "Nothing! I'm just helping out my slutty baby brother, just like the old man says I should. See?"
Kaigaku slips a hand down Zenitsu's front, finds his humiliatingly hard cock, and gives it a squeeze.
"Look at you," Kaigaku says, his voice a pantomime of concern, his eager thrusts giving the lie to his every word, "You got fucked to sleep just last night, but already you're hard and desperate for cock again. God, if I didn't take it upon myself to fuck your useless ass every day, you'd probably spend all your free time begging for cock on the streets, you whore."
Zenitsu shakes his head, doesn't know why he bothers, but something in him still tries to reject Kaigaku's words even now, even though it's pointless.
Kaigaku doesn't care, just keeps fucking Zenitsu hard, his thrusts fast and selfish, his hand on Zenitsu's dick just a ploy to further his humiliation by making his body react.
"It's better like this," Kaigaku whispers hotly, nosing the hair and shirt collar away from the back of Zenitsu's neck. "Like this, I'm the only one who has to know what a disgrace you are, little brother, isn't it so nice of me, to take on a burden like you? Why don't you say thank you, Zenitsu? Tell your big brother you're grateful."
"You're sick," Zenitsu cries out, the words forcing their way out of his mouth no matter how stupid he knows it is to react, how pointless, "You're so sick! How do you keep getting worse?!"
Kaigaku bites the back of his neck, hard. Hard enough to break skin, probably, and the pain of it shocks him enough to shut him up. Kaigaku keeps his teeth buried in Zenitsu's neck as his thrusts speed up, selfish animal abandon in every move. Zenitsu can hardly see, he's crying so hard, but he's quiet now, can't think past the pain in his body, in his heart.
When Kaigaku finally comes, it's with a pleased sort of growl as he pumps Zenitsu's ass full of hot cum, keeping himself buried deep inside all throughout.
"Let's try this one more time," Kaigaku says at last, forcing Zenitsu to turn his head, to look at him. There's blood on his lips. He really did break skin. "Say, 'Thank you big brother for taking care of me.'"
Zenitsu stares hard at the pink tinge of Kaigaku's canines. Says, hollow and flat, "Thank you big brother for taking care of me."
"Good boy," Kaigaku mocks, presses a sloppy, coppery kiss to Zenitsu's mouth. Zenitsu is still as Kaigaku pulls out, carelessly pulls up Zenitsu's underwear and pants, slaps his sore ass.
"Well, better get a move on then. Can't be late for school now, can you, Mr. Morals Committee."
Kaigaku chuckles to himself as he walks away. Zenitsu waits until he can't hear the shuffle of his bare feet on the floor anymore before he closes up his pants, wipes the tears from his eyes. He has no idea what to do about the blood on his neck, but if he doesn't leave now he really will be late.
Zenitsu opens the door, steps out into the free air, and breathes.