The Thoughts had been there for as long as Izuku could remember. They were a constant murmur just beneath the surface as he observed things, people, situations...everything.
The Thoughts themselves were always changing, rattling off quirk analysis and predictions and threat assessments and psychological breakdowns until Izuku felt like he would drown beneath the sheer amount of data he was constantly inundated with even when walking down a mostly empty street.
It was terrifying, and overwhelming, to look at the world and instinctively know how to break it down to its smallest, weakest pieces.
The Thoughts made him smart, helped him make and execute successful plans on the fly, made him a better hero.
But they made him a monster too.
Family issues, domestic abuse, trauma, sexual orientation (whether realized or not), psychology and moral compass: Izuku not only picked it apart, he turned it inside out. He weaponized it.
And God, did it make him feel guilty sometimes. But not guilty enough to give up the advantage the Thoughts gave him.
It didn't take Izuku long to figure out that not everyone had Thoughts like his, that other people looked around the world with wonder and awe while Izuku looked around with cynicism and knife-edge focus. So, Izuku kept the Thoughts quiet, kept them locked in his brain until they overflowed and escaped him in a manic mumbling mess, even as he put the Thoughts into practice evading bullies and Kacchan in Middle School and completing Practicals and surviving Villain Attacks at UA.
Meeting Yaoyorozu Momo was like a revelation.
The first time he met Yaoyorozu, he recognized something in her, in the way she looked at the world. She looked around like she was seeing how things fit together, like she could see the molecular and elemental blueprints of the things and people around her, like she was figuring out how to take them apart and put them back together again , and something in Izuku sat up and took notice of the Class Vice-Representative.
The first time he noticed her hands twitching for a pencil when she heard him mumbling, the monster inside of Izuku bared its fangs and grinned. She didn't seem aware of what she was doing, of how her fingers flinched toward a notebook or scrap paper the moment Izuku started giving voice to his Thoughts.
Slowly, he started encouraging her by accidentally sliding spare paper toward her while he mumbled, by just so happening to have an unused pencil nearby. And slowly, so slowly, Yaoyorozu started writing her Thoughts down as Izuku mumbled his Thoughts away.
Every monster needed a playmate, after all, and Izuku was determined to make Yaoyorozu his .
Once he had her hooked, he changed tactics slightly: incorporating more rhetorical questions, staring off into Yaoyorozu's direction, subtly adding in more psychological and moral focused questions as he went. He saw her pause, saw her contemplate, saw her sketch ideas out on scraps of paper and then he brought out the big guns: he started asking her questions in the midst of working out his Thoughts.
The questions started simple: did she agree or disagree with an idea? Did this Thought or that Thought make more sense? He deepened the questions over time, involving Yaoyorozu in hypotheticals, getting her opinion on strategy, weaponizing her nascent talent into something that could give Izuku a real challenge someday.
Eventually his solo Thought sessions turned into lively Thought debates with Yaoyorozu giving as much as she took and the monster inside Izuku howled with glee. He wasn't alone anymore! They could be monsters together !
Unfortunately, that feeling of belonging only lasted until the next Villain Attack on Class 1-A. Izuku and Yaoyorozu put together a plan that absolutely decimated the villains, leaving only smoke and fire and blood behind as the villains turned on their comrades and ignored their previous objectives. It was chaos, it was carnage, and at the end of it all the villains were so critically injured that the ones left awake couldn't even consider escaping. Izuku's smile was more like a baring of teeth as he watched his classmates trickle to safety in ones and twos, not a single injury among them.
Good! the Thoughts asserted, smug satisfaction spilling from Izuku as they ran numbers and probabilities as the likelihood of the next Villain Attack happening soon dwindled as the fires smoldered to blood-soaked ashes. It shows the villains right. They should never have tried to mess with what was theirs .
Izuku felt more than saw Yaoyorozu collapse to her knees beside him, eyes locked on the image her pristine hands made against the bloody, scorched earth the villains' dogfight had left behind.
"Oh, God," she breathed in horror, eyes wide and unseeing. Izuku crouched next to her, eyes flicking over her face, as Yaoyorozu continued. "I'm a monster."
Izuku sighed and dragged a spotless hand through his hair. "Yeah. You are," he agreed and Yaoyorozu's head snapped toward his, devastation written all over her features.
He reached out and slowly rested a hand on her shoulder, his green eyes trapping hers with manic intensity. " We are," he corrected, and Yaoyorozu's eyes fluttered shut as tears cut silvery tracks down her cheeks.
"I…I don't want to…" Yaoyorozu trailed off, and Izuku nodded slowly before collapsing beside her, eyes flicking between the battleground and her own.
"You can't stop being a monster," he stated, leaning his shoulder against Yaoyorozu in solidarity. Yaoyorozu shuddered as warmth spread through her from the contact. "You just don't have to be the evil kind."
A sob ripped from her throat as Yaoyorozu pressed into the contact, her shoulders hitching as she came to terms with what she had become, what she always was. Eventually, her sobs petered out and the twin monsters gazed over the devastation they had orchestrated together.
Two monsters, two friends, two heroes . And even if the Thoughts will never go away, they don't have to consume them. They can choose, Yaoyorozu can choose, every day, to do good with them instead of evil.
That would be enough. It had to be enough.