I don't own Katekyo Hitman Reborn or Blue Exorcist. Enjoy.
Prologue
"Look, anata. Aren't they beautiful?" Sawada Nana's eyes were brimming with tears of joy as she looked at the newly birthed twins she held in her arms.
"Yes, yes they are. Have they been given names yet?" Her husband, Sawada Iemitsu, awkwardly picked up the male one of the twins.
Nana shook her head no. "I thought that you would like to name one of them."
"Then…" He thought for a while before grinning and looking at the boy he held. "This little one shall be Ieyasu."
Nana chuckled. "You're naming him after your grandfather?"
"Why, yes. My grandfather was a great person. I'm sure my son will be the same!"
Nana giggled. "I'll name my baby girl Tsunahime."
"You're always one for strange names."
"I am not!"
"Oh, really? What about the time you named our cat Ietsuna?"
"I like shogun names, ok?"
"So you say." Iemitsu frowned when he thought of something. "By the way, I read about how long pregnancies usually take. How come you were two months earlier?"
Nana shifted. "I'm not sure, but I had a strange dream last night. I saw a boy with pistols shooting at something, then a girl with Satan's flames holding a sword. Next I saw the same girl with the same flames as you, and then blue flames appeared. That was when I woke up."
Iemitsu hummed in thought. "A girl with the same flames as me… I'm pretty sure I'm the only human with Sky flames. The only other one I know that have sky flames is Luce."
Tsunahime cried, a small sunset-orange flame flickering into existence in the middle of her forehead. The two parents gasped.
"Did you see that?" Iemitsu asked after the flame spluttered out a moment after. Nana nodded slowly, then looked at Ieyasu, wondering if he also has the flames his sister possessed. "Does he…"
"Also have sky flames? It seems so." Iemitsu said after a moment.
Tsunahime cried again, this time louder. Two tiny blue flames appeared on either side of her head, flickering out a second later.
"Satan's blue flames? But how?" Nana wept.
"I don't know, but we'll have to seal it away. I'll call Fujimoto." Iemitsu gave Ieyasu back to his mother, then went outside to call his friend.
Nana stared at her son. "Be a good brother, alright?" Ieyasu squealed, as if answering her.
10 minutes later, Fujimoto came and sealed Tsuna's power into Kurikara, a demon-slaying sword.
5 years later, Ieyasu showed signs of being able to see spirits and Tsuna started taking fighting lessons from Hibari Kyoya: tonfa user, someone who has a soft spot for cute things.
6 years later, Iemitsu and Fujimoto trained Ieyasu the ways of exorcism so he would be able to protect Tsuna. Tsuna befriended Gokudera Hayato: dynamite user, Yamamoto Takeshi: baseball star, and Sasagawa Ryohei: boxing captain.
8 years later, Nana died of sickness. Ieyasu became the smartest person in his grade. Tsuna became friends with Bovino Lambo: grenade lover.
11 years later, Iemitsu died in a car crash and the children now lived with Fujimoto. They still attend Namimori Elementary. Ieyasu skipped a grade. Tsuna befriended Rokudo Mukuro and Dokuro Chrome: illusionists and trident users.
13 years later, Ieyasu began studying to become a doctor. Tsuna and her friends created a vigilante group call Vongola. Namimori Town was cleansed of gangs within two months.
15 years later, Ieyasu received a scholarship to True Cross Academy. Tsuna's friends also moved there. Tsuna got a job at Takesushi.
And the adventure begins…