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If there was one thing in the world that Tenko cared about it was his Sensei. The man who had saved his life and took him in when no one else would. The man who had taught him everything he knew. At thirteen it was hard for Tenko to remember a time before Sensei. After all, he'd just been a baby when his Father had been killed. Just a snot-nosed, ugly, little kid who no one looked twice at, who no one thought was worth saving.
Tenko couldn't remember his mother, well, there was no way he could, he'd killed her as she gave birth to him. The dangerous Quirk of an infant taking the life of the very person who gave him life. His Father, on the other hand, he did have some memory of. A cold, aloof man who would lock him in his room for days on end, pushing cold oatmeal through a cat flap in the door to feed him.
He couldn't look at Tenko, couldn't touch a monster like him. Tenko had always thought that maybe he could prove to his Father that he could be a good boy. That he didn't have to hide him away. But the rare times his Father let him out he always managed to do something to get himself locked back up again. Alone in his dark room, that smelled like an unwashed little boy and human excrement, desperately swinging between wanting his Father to love him and wanting to wrap his tiny hand around his arm and watch him crumble away.
But now he had his Sensei. His Sensei cared about him and was kind to him. He was teaching Tenko how to take care of himself and about the world and about his past. Sensei knew Tenko was smart, and he knew Tenko wanted the truth, and he provided. Sensei told him all about his family history.
He told Tenko all about the woman, the Hero, Shimura Nana. How she had carelessly and ruthlessly thrown her son away to keep him out of her way as she collected and stored power for herself. Her son, Tenko's Father, hadn't known any love himself as a child so how would he have been able to show Tenko any love? Senesi told him how Nana had a student, a Hero everyone, even sheltered Tenko, knew about. The Symbol of Peace, All Might. Sensei told him how All Might had stolen Nana's power and gotten her killed. That All Might was a thief and a villain that no one knew about. He told him that All Might had destroyed his family. That it was All Might and Shimura Nana who had gotten his parents killed. That Tenko was innocent. But, if Tenko was willing to break a few eggs, he could have revenge on them and the society that made them.
Sensei taught him daily. Be it history lessons or physical training. Tenko was an avid student with a sharp mind and quick wit, he picked things up quickly. He made his Sensei proud. Sensei would always shower him with praise when he did well. It made Tenko feel good, happy, the best he'd ever felt. The teenager lived to hear,
"Well done, Tenko, my boy!"
But sometimes he didn't do so well. Sensei never locked him away. He never hurt him. Though Tenko lived in fear he might. But the look in his eye and the tone of his voice left more lashes that a whip ever could.
"Try again, Shimura."
Shimura. Sensei always called him that when he failed. He dropped the affectionate "Tenko" and threw the name of the woman who had abandoned him to abuse at him. Shimura. The name of a coward. Shimura. A name of inadequacy. Shimura. The name of a fake.
Tenko couldn't bear to be associated with it. It put him over the edge and drove him to madness. It made him want to cut himself out of the very skin he wore, rid himself of the DNA they shared. He would scratch. He tore at his body with ragged fingernails. He never clipped them. The longer they were the more damage they did and the further away he could distance himself from that hated name. Shimura.
He couldn't take it much longer. The more he obsessed about it the more distracted he became and the more often he failed the challenges Sensei set for him. It was rare he was ever called 'Tenko' anymore. It didn't even feel like his name at this point. His only name was Shimura. Shimura. Shimura.
Weak.
Disgusting.
Failure.
It wasn't long before his young mind broke.
He was knocked on the floor of the training room by Sensei mid sparring battle when it happened.
"Get up Shimura, that's not what I taught you."
It was the last straw. The drop of water that created a waterfall. The piece of kindling that turned embers into flames.
Tenko shot up from the floor and lunged at his Sensei. With no wasted movement born from years of martial arts training, he masterfully executed the takedown they had been working on. With Sensei on his back, Tenko lept on top of him, drew his arm back and punched him in the jaw. He pulled back and struck again and again and yet Sensei, with all his power, did nothing to stop him. He just started laughing as Tenko wailed his anger against his face. Tenko couldn't take the laughter. He stopped his blows and wrapped eight fingers around Sensei's throat and squeezed. Sensei stopped laughing and their eyes met. Crazed crimson facing down deepest black.
"Don't call me that." Tenko spat. His extended index fingers wavering. Without leaving his eyes his Sensei began to sit up and Tenko, belatedly realized what he had done. He quickly pulled back his hands and scrambled off the man, retreating to the corner in the stance of a fighter, ready for his punishment to be unleashed.
But nothing came. His Sensei stepped forward and brushed a large hand through his hair, a smile on his bleeding face.
"What would you like me to call you?"
Tenko looked up into his teacher's eyes. He wasn't angry. Did he look...proud?
"Shigaraki."
Death grip.
"Tomura."
Funeral.
"Excellent." His Sensei hummed. "Well done, Tomura my boy."