Disclaimer: I don't own My Hero Academia or Undertale.

Rule Four: All's fair in love and war. Deals are a must.

There were a lot of things Izuku knew about Katsuki Bakugou. After all, he took notes on everybody he found interesting, especially if he found a particular Quirk he liked to analyze. He knew the other boys likes, dislikes, fears, how he used his Quirk, its limitations, and more.

Hisashi Midoriya had said something that resonated with young Izuku. The man had whispered a quote to him before he wrapped his arms around a giggling Inko. All's fair in love and war.

Normal kids wouldn't have taken the words at face value. Nor would they give the idea that he could do anything as long as he didn't get caught.

He practiced everywhere he could. He practiced on school bullies besides Katsuki because he knew the other kid was just as smart as him and would notice the subtle pranks. Tied shoes made bullies trip. Books, pencils, pens, and other school materials slowly moved to the edge of desktops and small bumps made them fall getting the teachers ire for disruption of classes.

He practiced at roadblocks, where Villain and Hero fights happened in front of him. He had learned to put his hands deep in his pockets and he moved small objects into the path of Villains without anyone's notice, as if he used his Quirk in short bursts, that there wouldn't be the telling sign of glowing green.

Then their was his favorite practice material. Hisashi Midoriya. The man who came back only because the boy wasn't Quirkless. The man who thought he could just barge back into his life. The man who was clearly not good for his mom.

Izuku watched Hisashi Midoriya and the man knew it.

Inko was happy that the man was back. They went on family outings. They talked about Quirks. They discussed Heroes and Villains. They went shopping for figurines and clothes.

Inko had no idea that her son unnerved the man with a glowing eye. Or the venomous puns and jokes laid his way. Or the way Izuku smiled when Inko's back was turned. The wide grin reminded the man of the Symbol of Peace: All Might. Except Izuku's smile did not give hope. The boy was judging him.

Hisashi Midoriya was afraid of his son.

At first it was just that the boys attempts at jokes and puns that were unnerving. The more the boy used his Quirk to intimidate him, the more he felt an unknown weight upon his body, and the eye... That eye felt like it was piercing his very being. As though it could read the things he had done.

Unfortunately for Hisashi Midoriya, he was exactly right. Izuku had learned to see a persons soul through continuous use. Every soul could be read. He could read the sins of a person...

Hisashi Midoriya's list was long.

Izuku knew he needed practice with his Quirk before he could do anything to the man he called his father.

So in came Katsuki Bakugou and the notes. The first person he would completely trust. He knew exactly why the other boy called him Deku and tried to beat the former Quirkless boy down. He also knew he was loyal to those he could stand to to toe with. Izuku needed to know Katsuki was on his side.

Katsuki Bakugou wanted to be the best Hero ever. He couldn't be a Hero if his friend got killed without a way to defend himself. If someone beat Katsuki, then his friend wouldn't have a chance.

Katsuki Bakugou was a test for Izuku Midoriya. To have an ally at his back. He would have his former best friend back and someone he could rely on with learning to control all of his Quirk. Izuku would have to abandon his first rule if he wanted to master his Quirk. So, he altered the first rule. Only use the full extent of Quirk on a Villain or during a spar with permission from an opponent.

Izuku motioned for Katsuki to follow him.

"My Quirk is a secret Kacchan. I need help training it."

Katsuki looked at him in confusion. "Why do you need my help Deku? I'm strong. Your weak."

"I'm not weak Kacchan. But, I need to get stronger than my dad." Izuku frowned as he mentioned his dad.

"What's wrong with your old man?"

"Trust me when I say you don't wanna know."

"No. I think I do wanna know Deku. Otherwise I won't help."

"Okay, but that means you can't tell anyone without permission."

"Deal."

Then Izuku grinned. "Thank you Kacchan."

Izuku's pupil glowed emerald and the light turned into wisps that curled like a tongue of flame. Katsuki felt a chill run down his spine. Izuku's smile was wide and let his teeth show. The shade of the trees seemed to darken the curly haired boys hair and cover half of his face to make him look menacing.

Katsuki Bakugou wouldn't know until years later that it was at that moment he learned the expression "To make a deal with the devil."