How about some per-canon stuff?
TW: violence, bullying, and references to drowning
Pyro grimaced as the grate screeched. It was too heavy to not drag across the pavement when he wanted it gone, so he could only hope no one heard it.
The sewers seemed like the only safe place sometimes. Father wasn't there to bore his eyes into the back of Pyro's head, mother couldn't sigh like she expected the failure she decided he'd become. No nannys who scolded him for not sitting right in his chair. The sewers were quiet. He liked that.
He didn't mind the water lapping at the edges of the edges of the walkway. He wandered deeper and deeper until he found a pile of metal crates stacked haphazardly in the corner, next to a ladder that led through one of the manholes in the road.
"I'm here, you can come out now!"
Seemingly at the tone of his voice, a rat skirted out from a hole in the wall and hopped onto the boxes. It squeaked high-pitched at him.
"Impatient much?" He chuckled as he reached a hand into his pocket. He pulled out half a loaf of sourdough bread, ripping off a piece and handing it to the rodent.
"Just you today, Fillochini?" The rat squeaked again, nibbling at the chunk of bread he'd been given.
Rats. Sometimes it seemed like they were the only creatures that could understand Pyro. He'd been feeding the ones that lived in the sewers for almost two years now. Teaching them tricks, and stuff.
Pyro pet Fillochini on the head. He liked the sewers. Nothing but squeaking rats and lapping water. Stone walls that covered everything horrible about his life.
"Ha! And here I thought you couldn't get any more pathetic!"
Pyro flinched at the voice behind him. Fillochini did the smart thing and ran back into the hole in the wall. Pyro didn't have any holes in the walls. All he could do was whip around to face the one thing stone walls failed to block out.
"Feeding rats?" Czesław chided. "Makes sense. Maybe the reason you're so daft is because these sewers left some fumes in your brain."
Pyro's entire body shook. He stuttered, panicking as he tried to get as much distance between him and Czesław as possible. Then his back was against the metal crates behind him, and he was trapped.
Czesław closed the gap, grabbing Pyro by his shirt and slamming him face-first into the box. His hand gripped his hair as he leaned in close.
"Maybe if one of those vermin bites you you'd keel over and die already like everyone wants you to."
Pyro sobbed. His face was wet, and he realised in horror that it wasn't just from tears. The impact with the crate had broken his nose. He screamed, but it was only muffled into the cold steel he was against.
Czesław scoffed. He pulled Pyro away from the crate, shoving him to the floor instead. Pyro tried to get up, but he was stopped by a foot pressing into his wrist.
"Why do you try fighting back when you know it doesn't matter?" Czesław looked at him like he was telling him which foot was the left one, "I don't even need to wait for the rats and fumes to kill you. I could do it myself. Honestly, you should be rewarding me for holding back all this time."
"Czesław, please just le-" Pyro's words cut off into a strangled scream as his assaulter put all his weight onto the foot that trapped his wrist. A dreadful crunch rang through the sewers. They were too quiet now.
"In fact," Czesław let rage fill his expression. He dragged Pyro across the concrete, letting his head and shoulders dangle over the edge only inches from the waters surface, "I could just hold you down in the water, see how long it takes for you to go limp."
Czesław laughed lowly as Pyro struggled and screamed and so bed under him. He drank in the sight of it for a long time, before pulling Pyro back from the edge. He dropped him onto the ground with an unpleasant thump.
"But, alas, I'll let you live. How gracious I am." Czesław gave Pyro one last wild grin before turning anfld walking away. Pyro laid on the ground, letting himself cry for a minute. Then he got up and went home.
He made up some excuse for the injuries, like he'd done a thousand times before. He sat at his desk for the entire night. He dove into his studies, trying to get his mind off of the experience. He nibbled on the sourdough bread he still had in his pocket.
Pyro never went back to the sewers after that.