Back hundreds, thousands, millions, billions, etc, of years ago, Nega and Nazo weren't as they are today. Nega was a normal nine year old girl named Tenshi Rosuto and Nazo was an average mobian hedgehog, where he had no actual name but was nicknamed Nazo, Japanese for mystery, by Tenshi. Tenshi was the only child of her parents, though ever since finding Nazo one day when she was young, she had thought of him as a brother.
Now, Nazo liked exploring, and finding treasures hidden around, and one day he came home from one of his mini-adventures carrying a certain set of seven emeralds, the Chaos Emeralds. He took them to the dining room table and showed them to Tenshi, as her parents were out at the time.
He had explained what they were, their power, and how amazing it was he'd been able to find them, and was very happy with himself. Tenshi asked what he was going to do with them, and Nazo replied "I'll keep the for myself!". This left Tenshi confused. Nazo was well know for donating all his finds to either charity or museums, so for him to so quickly declare he was going to keep them for himself was odd. She asked if he was feeling okay, and he replied "Of course I am!" in an angered tone. She decided to leave him alone for the rest of the day after that.
That night, she awoke to noises from downstairs, almost like fighting. Frightened from the possibility of someone in their house, she snuck into Nazo's room to wake him, but found his bed empty and various parts of his room in pieces. Assuming the worst, she was about to run to her parents room when she heard a scream of pain from downstairs, sounding like her mother, before it quickly fell deadly silent.
A few moments later, from downstairs, she heard a laugh. A laugh of enjoyment from the sick monster who was doing this. But what disturbed her wasn't the laugh but rather whose laugh it was.
It was Nazo's laugh.
Confused and absolutely terrified at this point, she ran to a window and looked outside, for a way to get out, but she was much too high up to jump. Footsteps could be heard coming up the stairs, and her heart leapt into her throat.
She crawled behind his bed, and curled into a ball, hoping he wouldn't find her.
A few moments later, the bed was lifted up by a single gloved hand and thrown against the wall, smashing to pieces. Standing there was Nazo, smiling maliciously, his body distorted by dark power. His all of him except his gloves, socks and eyes had turned a pulsing jet black, and his eyes no longer had pupils. He reached down and grabbed her, lifting her up to his height.
Tenshi was sobbing hysterically at this point, unable to control herself, and had even wet herself in fear.
Nazo chuckled with evil intent, before explaining every detail about how he had slaughtered their parents, and how he had intended to do the same to her.
She managed a look at him. Intended?
He chuckled once more, before explaining how instead of killing her, he was going to transform her into a being like him, manipulated and distorted by the negative energy of the emeralds.
She screamed and struggled in fear as he carried her down the stairs, pushing the door open, revealing the living room nearly completely gone and two small piles of ash on the floor. He punched her in the chest, knocking the wind out of her, making her cough up blood, and turning her screams into whimpers.
He strapped her to the table and lifted three of the emeralds, now pitch black, over her. Darkness began to flow from them into her, her body grew cold, and everything went black.
After that, Tenshi was no longer. The girl of darkness woke up, her hair and clothes stained black with darkness, her eyes turned white but the pupils still there. She sat up and looked around the destroyed dining room.
She broke into a fit of giggles at the destruction, before deciding that she wanted to copy this everywhere she could. But first, she thought, she needed a name. She didn't know where it came from, but a thought entered her head.
Nega would be a good name, she thought, and decided she'd call herself that. She climbed off the table, the straps snapping, and walked out the back door. She looked down at Tenshi's pet cat, giggled, and it exploded into darkness.
She flew upwards, and looked around. Most of the city seemed untouched, though there was a trail of complete destruction in one direction, where Nazo had gone. She flew that way, and landed nearby him.
Nazo chuckled. "So, how do you feel?"
She giggled. "I feel good! Can I join you?"
He nodded, and she grinned, before aiming a sphere of dark energy at a mother and her child. She fired it, but it was deflected back at her by a flash of green.
A girl was now standing between her and the mother, looking no more than thirteen. Nega and Nazo glanced at each other, they didn't know how, but they knew this girl. The Green Chaos Emerald.
She stared them down.
"So, this is where our negative energies went. With no way to take it from you without taking out a large chunk of the multiverse..."
Both of them cried out in alarm, their bodies beginning to dissolve into a dark mist, being sucked towards her.
Six more girls appeared surrounding her, the six other emeralds.
For the briefest moments, Nazo's eyes regained their pupils and he stared at Nega. He mouthed "Tenshi... I'm sorry..." before they were sucked and sealed into the Chaos Emeralds, for all of eternity.
After that, the emeralds faded away, restoring the city to how it once was. the only difference was in one house, where a married couple had tried to have a baby, but couldn't. They would have named her Tenshi, but that was not to be.