So this part is just to show the setting of this world slightly. It's a little hard to grasp but with time everything will fall into place. Trust me.

*Fades in*
"Your sure that this will stop the war between Humans and Monsters right?" says the mysterious man.
"Dragons are the source of all life, especially the Fire Dragon. If there is a child with his powers but also human everyone will always follow him. It will stop our war." A white dragon in human form speaks.
A woman in pure white robes comes out from behind the man from before, "If this will work then, I will have the child to stop this war. To many lives have been lost thanks to this war. Human and monster alike."
"Well then, prick your finger and sign the magical contract. The Fire Dragon has already agreed and signed."
She walked forward and pricked her finger, signing the contract.
*Fades out*

Months later, the perfect child is about to be born when the dragons realized that not only was the Fire Dragon dying, after transferring all his powers to the child, his wife, the Ice Dragon, was dying too. During that time, the human noticed that the "perfect child" was more than just one child. Everything didn't seem to go as planned.
When the "perfect child" was born, they found out that the one child was in fact twins. Twins with uncontrollable powers. The girl, accidentally burned the nurse alive. The boy, accidentally froze the doctor to solid ice. "This is not according to plan! It was to be one child not two. The child was to grow into the powers as well! This is not right! We have to go to the dragons after we hide these two. I'm sorry dear, but we have to run now." the man from the first scene says. He grabs his wife, sliding her into a wheelchair and places the two babies into her arms, covering them with cloth to hide them as they leave the hospital. "If we make it home we should be safe there."

A year has passed and the children grew faster than humans. Both of them were already around the human age of 7 years old. The parents of the "perfect children" decided that for the sake of humanity and the war, the fact that the one child is two should remain a secret. Even though they decided that the dragons already knew it was two, seeing as how two dragons died on the day of the children's birth. As they grew the parents knew the children were going to be extremely special. They were only 1 year old but they knew how to read and write. The girl could speak fluent English already. The boy breathed ice every time he tried to speak so he kept his mouth shut. He therefore spoke though his sister to the world what he wanted to say. They named the girl Yuki Onna, after the Japanese snow fairy. The boy they named Kaji, the Japanese for fire. They thought it was funny to give the children names of the exact opposite that they were. They were both locked in a room with hundreds of books that the parents gave them to read. The parents were afraid that with the uncontrolled powers those two possessed that they couldn't control the children. If they couldn't control the children then locking them up was their next choice.

Unfortunately the father never knew what the mother was doing to them while he was away. The mother, upset she couldn't have the perfect child, started training Yuki to have children. Even though the child wasn't able to, the mother didn't care. One night it went to far for Yuki. Yuki started screaming for her mother to stop even though she wouldn't. Kaji, feeling his sisters pain, ran into the mothers room. Kaji froze their mother in ice and grabbed his sister away. Yuki realized that when their father came back Kaji would be punished. She had to protect him. She thawed their mother and laid her in her bed. When the father came home, he found his wife in bed still cold. He confronted the twins about why their mother was cold. Yuki spoke up.
"Father, she was coming into our room and startled me. I started to burn her by accident and Kaji stopped me. He froze her a bit trying to stop the flames around her. I'm sorry. If anyone is to blame it is I."
He looked at Kaji, and instantly knew the truth. Kaji could still not control his powers while Yuki was perfect with it. There was no way Yuki would have done that. He also realized that if Yuki was taking the blame than he couldn't punish Kaji. He decided to split the twins up.
"Yuki I can't trust you in this house if you set people on fire. I know we were trying to keep you two secret but as long as Kaji is here, you can not spread word that you are not human. Understand?"
"Yes father. I will leave this house. I know we both can't die from anything already so I will live alone. I will keep the secret as long as you care for Kaji. Thank you father for everything you have done."
Yuki grabbed her sole spare set of black clothes and left the house. She didn't even turn back. The twins father locked Kaji in his room, planning to never open it again. Thus started the life of the twin immortal dragon/human children.